Pearl glossary

Abalone pearls Natural pearls or cultured pearls produced by the saltwater pearl-producing mollusk “Giant Abalone” (Haliotis-Gigantea). They are horn-shaped pearls, with multicolor reflexes and high luster.
Acephalous See bivalve.
Adductor Muscle of the bivalve mollusk that unites the inner part of each valve. It opens and closes the mollusk allowing the adduction movement.
Akoya pearls Saltwater cultured pearls produced mainly in Japan. The good dimensions, the soft colors and the extraordinary luster have made them the most popular, elegant and versatile in the market.
Akoya-Gai See Pinctada martensii fucata.
Alasmodon Margaritifera Freshwater bivalve pearl-producing mollusk. It is found in the rivers of Canada and occasionally produces calcitic natural pearls (Alasmodon pearls) with a radiated structure.
Aloha pearls See Shell pearls.
Ama Japanese female pearl divers. They were trained to fish Akoya-Gai oysters and were active in Japan up to the mid 60’s. These fisherwomen (in Japanese Ama means “sea daughter”) would dive without scuba gear or air tanks in quite deep waters for long periods of time.
Angelo pearls See Shell pearls.
Anodonta Cygnea (Swan mussel) Freshwater bivalve pearl-producing mollusk of the family Unionidae. It achieves up to 20 cm in length and has a rather thin shell. It occasionally produces natural pearls. In 1852 they were described by the Italian naturalist Filippo De Filippi, who distinguished an organic nucleus produced by a Trematode (a parasite).
Apex See Umbone.
Aragonite Crystal form of calcium carbonate (CaCO3) characterized by a structure of microscopic pseudo-hexagonal forms. It is a high pressure polymorph of calcite and crystallizes in the trimetric group, rhombic system, with a long prismatic crystal or in trigeminals that simulate hexagonal prisms. In pearls the aragonite cannot be formed by the activity of high pressure. So it is thought that the chemical environment helps the precipitation of a kind of calcium with a different coordination. In fact, in a sedimentary environment the aragonite is easily formed in metastable conditions, helped by the biological activity or the chemical precipitation of ionized solutions like lead, zinc and strontium.
Australian pearls Term used to refer to South Sea cultured pearls, which are produced mainly in Australia.
Awabi shells See Haliotis-Gigantea.
Azra The most famous Tahitian natural pearl. It is a pendant attached to a necklace belonging to the Russian Crown.
Baroque pearls Cultured pearls or natural pearls that are not round and have an undefined, irregular shape, and also said ‘scaramazze’. They were very used in jewelry during the Renaissance period. They are still very appreciated in the production of artistic jewelry, characterized by a creative design.
Binary Term related to diffraction diagram or diffractogram or Laue-gram, in which we can see four dark central spots surrounded by six or more external lighter spots. We can see that by X-raying an aragonite geminate. Binary diffractograms are typical in cultured pearls.
Biological See Organic.
Byssus A silky filament by which certain mollusks attach themselves to hard surfaces. In the past it used to be worked up to becoming golden and silky. It was used in luxury accessories with the name of “sea silk”.
Bivalve A mollusk from the class Bivalvia having a two-valve shell attached by a hinge.
Biwa-perle Definition attributed to the first cultured pearls without a nucleus (more precisely an organic nucleus) obtained in the Biwa Lake (Japan) in 1924, from the freshwater pearl-producing mollusk Hyriopsis-Schlegeli (Biwa pearl mussel). The gems are flat and long like a stick, due to the graft of a long fragment of mother-of-pearl. The production grew on a large scale from 1970 to 1980. That is why initially the name Biwa was attributed to all freshwater cultured pearls.
Button pearls Flat, button-shaped cultured pearls or natural pearls. They are particularly appreciated for earrings and rings as they are less prominent and more discreet.
Blister pearls Hemispherical pearls because formed between the mussel and its shell and not on the mollusk tissues. Once they are removed and worked they will be called Mabe pearls.
Bohemian pearls Imitation pearls. They are irregular little spheres made of shaped mother-of-pearl protuberances. They are mostly commercialized in Eastern Europe.
Bu Ancient Japanese unit of measurement still used to measure the diameter of rigid nuclei in mother-of pearl: 1 bu = 3,03 mm.
Black-lipped oyster See Pinctada-Margaritifera.
Bleaching Chemical treatment used to lighten the color of pearls.
Black pearls Black or very dark pearls from Tahiti. They have very high iridescence. Black natural pearls are extremely rare.
Biological product Material of organic, inorganic, or mixed (inorganic and organic) composition formed by living organisms. Pearls are biological products.
Calcite Stable crystallized calcium carbonate (CaCO3). Together with aragonite they constitute the mother-of-pearl and pearls.
Carat Unit of weight that is very used for all gems. 1 carat (symbol ct) corresponds to 0,20 grams. 1 gram = 5 carats.
Calcium carbonate Organic substance (CaCO3) found in rock in all parts of the world. It is the main component of pearls like aragonite and calcite.
Carotenoids Organic pigments of animal and vegetable origin. They are yellow, orange, red and are the ones responsible for the rosy color of natural pearls and cultured pearls. Carotenoids get decomposed if exposed to the ultraviolet rays of the sun light. Hence even rose pearls tend to fade if exposed to the sun light for a long time.
Cephalopod Univalve pearl-producing mollusk. Its name derives from kephalE’=head and podòs=foot, that means mollusks whose feet are in their heads. Among mollusks these are the most evoluted species. The sea genus Nautilus (family Nautilidae) is the only gender with an external shell and it occasionally produces pearls.
Chinese pearls Cultured pearls produced in China with or without a nucleus and obtained from the freshwater pearl-producing mollusks Hyriopsis-Cumingii (“Triangle shell mussel”) and Cristaria-Plicata (“Cockscomb pearl mussel”). They are commercially defined as Freshwater pearls. Recently in China they produced cultured pearls with a rigid nucleus by using the same mollusk that is used in the cultivation of non-nucleated pearls: the “Triangle shell mussel” (Hyriopsis-Cumingii). But such technique presents some difficulties and superior costs. Thus the Chinese pearls with a nucleus represent just a small part of the noticeable Chinese production (a thousand tons a year).
Choker A one-strand necklace that is 14-16” (35,5-40,5cm) long. It nestles just at the base of the neck.
Clam pearls Whitish or yellowish natural pearls produced by pearl-producing mollusks of the Pinctada or Unionidae family.
Collar Necklace 10-13” (25-33cm) long. It is usually made up of three or more strands and sits directly against the throat.
Color (of a pearl) It is composed by the base color and the additional color, which are the nuances produced by the diffraction of the light on the nacre.
Cultivation Artificial technique used by man to induce the production of pearls by using pearl-producing mollusks. It is applied by specialized technicians trained to graft oysters with irritant agents (small spheres of mother-of-pearl in saltwater mollusks and fragments of epithelium in freshwater mollusks), which are fundamental to stimulate the deposition of nacre. This term is also frequently used to indicate the installation where there is the cultivation.
Composite pearls A more suitable term to refer to Mabe pearls.
Chemical composition of pearls Natural pearls are composed by 82-92% of calcium carbonate (mainly aragonite), 4-14% of conchiolin (organic substance), 2-4% of water and small percentages (less than 1%) of various chemical elements (Mn, Mg, Sr, K, Li, Cu, Zn, Cl, P).
Conch pearls Natural pearls occasionally produced by the gastropod sea mollusk Strombus gigas (known as “Queen Conch”). They are rosy pearls of small dimensions and have a “porcelaneous” aspect characterized by an iridescent radiated flame-like structure. They are also produced by some Bivalves (“Pen Shell”, “Giant Clam”). Those produced by giant clams are white.
Conchiolin Scleroprotein that together with crystallized calcium carbonate (in aragonite and calcite) forms the shell of mollusks and the nacre of pearls.
Concretion (biological) A concretion is an organic or an inorganic or even a mixed aggregation. It is a strange body that is formed in an organism from mineral precipitations or sedimentation of materials. Cultured pearls or natural pearls are biological concretions.
Connective tissue Tissue with the function of connecting other tissues between each other in order to help their nutrition and to protect the organs. In the mollusk the connective tissue is an intermediate muscled part of the mantle, formed by smooth unicellular fibers.
Consistency In nucleated cultured pearls, the consistency is the thickness of the nacre layer produced by the oyster around the rigid nucleus of mother-of-pearl grafted by man. The more it is consistent, the higher is the value of the cultured pearl.
Cortez pearls Natural pearls occasionally produced by the saltwater mollusks “Black Lips” (Pinctada Mazatlanica) and “Rainbow Lips” (Pteria Sterna). They are opalescent and have irregular shape and relatively big dimensions.
Cockscomb pearl mussel (cristaria plicata) Freshwater pearl-producing mollusk of the Unionidae family used in channels and rivers in China for the production of freshwater cultured pearls. Compared to the “Triangle shell mussel” (Hyriopsis-Cumingii) it has a thinner and smaller shell. It produces smaller cultured pearls of lower quality. It can reach 15-20cm in length. When it is 8cm long it is used to produce non-nucleated cultured pearls.
Cultured pearls Pearls produced after the graft of a nucleus (“rigid” for saltwater cultured pearls, “organic” for freshwater cultured pearls) in the soft tissues of a pearl-producing mollusk.
Cleaning Care operation that should be done regularly on pearls to maintain their beauty. It is advisable to clean the pearls with a soft cloth after each use, before putting them away. Besides, it is advisable to clean the pearls more deeply from time to time, by rubbing them with a cloth soaked in sandal wood oil.
Coating Treatment that consists of coating the pearl with a very thin layer of silicone in order to smooth the rough surface of the pearl.
Coloration See Dyeing.
Chinese glass See Placuna-Placenta.
Dar Ancient Arabic term used for pearl.
Diffraction Optical effect in which light radiations when in contact with small objects or cracks, continue through them in directions that are different from the original ones. The pearl luster (and the mother-of-pearl luster) is caused by diffraction.
Diffractogram Radiographic sheet film that gets recorded in a pattern of reflections of an x-ray beam diffracted by the exposed substance. It is also called Laue-gram.
Diffractometry Analysis technique that uses x-ray diffractions of substances. The used methods are two: the Laue pattern (or single crystal) and the Debye, Scherrer and Hull pattern (or dust).
Dyeing Artificial treatment used to add colors to pearls by using coloring chemical agents or vegetable dyes, usually after the bleaching treatment.
Ectoderm See outer epithelium.
Endostraco Vedi mother of pearl.
Epithelium Mollusk tissue without any blood vessels. Its function is to cover the mantle and it is composed by the inner layer (inner epithelium) and the outer layer (outer epithelium).
Essence of Orient Sostanza artificiale iridescente utilizzata nella produzione di imitation pearls. È costituita da una sospensione di guanina (cristalli iridescenti prelevati dalle squame di pesce) in un liquido organico, solitamente nitrato di cellulosa.
Fiji-perle See Shell pearls.
Freshwater pearls Non-nucleated Freshwater cultured pearls produced mainly in China. These pearls are formed in various species of freshwater mussels of the family Unionidae, which are found in lakes, rivers, ponds and other bodies of freshwater. The great variety of color, irregular shapes and contained dimensions have made these pearls ideal for creative jewelry.
French pearls See Paris pearls.
Fading Gradual loss of the luster of a pearl.

See Paris pearls.

Freshwater cultured pearls Cultured pearls of oysters or other freshwater mollusks. The biggest freshwater pearl cultivations are in China.
Freshwater natural pearls Pearls produced spontaneously by freshwater mollusks. The most appreciated are the ones belonging to the genus Unionidae, in the rivers of Europe, Asia and America. The pearls produced in the Mississippi river are well known for their quality and beauty.
Gamanose shell Generic name given to the mollusks of the family Unionidae that live in the Yangtse river in China. The mother-of-pearl of the shell was used to produce the rigid nuclei of Japanese cultured pearls.
Gastropods Univalve mollusks whose name derives from gaster=stomach and podòs=foot, that is, mollusks whose stomachs act like feet. They have only one shell that is typically spirally coiled. Pearl-producing gastropods live in saltwater (Abalones, True conchs, Turban snails and Volutes).
Geminate Special case in which there is an association between more individuals of the same mineral species according to the gemination laws. Two crystallographic elements of the interested species are pooled. When it comes to pearls the aragonite crystals are always trigeminated, that is, they are composed by three individual crystals that are united to form pseudo-hexagonal prisms showing an apparent superior symmetry.
Gold pearls Saltwater cultured pearls that have a gold color. Gold pearls are South Sea pearls and are very prestigious, popular and expensive.
Grain Unit of weight used for natural pearls: 1 grain = 0,05 grams
Guanine Extremely small shining crystals present in the coating membrane of fish scales. They have been used since ancient times to compose the essence of Orient.
Graduated necklace 43cm-long pearl necklace that has a central bigger pearl followed by smaller pearls in decreasing sizes.
Gold-lipped oyster See Pinctada-Margaritifera.
Graft Introduction of a nucleus in the mantle of a pearl-producing mollusk in order to stimulate the formation of the pearl. To obtain nucleated cultured pearls (from saltwater pearl-producing mollusks) pearl farmers graft a rigid nucleus of mother-of-pearl and a small piece of epithelial membrane, known as parenchyma, able to become a pearl-sac. To obtain non-nucleated cultured pearls (from freshwater pearl-producing mollusks) pearl farmers graft an organic nucleus composed by very small sections of a fragment of the mantle of another mollusk.
Haliotidae Family of gastropod pearl-producing mollusks to which belongs the genus Haliotis.
Haliotis pearls See Abalone pearls.
Haliotis Gigantea Saltwater pearl-producing mollusk of the family Haliotidae, also called “Abalone”, “Awabi”, “Ear-shells” and “Sea Ears”. It lives in tropical seas and can reach dimensions of 25 cm in diameter. It has a wonderful iridescent shell (with infinite shades of green, gray, blue and purple) characterized by a series of aligned holes along the bottom of the last circle. It occasionally produces natural pearls that have the same shades as the shell, known as Abalone pearls or Haliotis pearls.
Haliotis Rufescens Pearl-producing mollusk of the family Haliotidae. It is known as “Red Abalone” and occasionally produces reddish pearls.
Hama-age pearls Akoya cultured pearls of superior quality. The really extraordinary pearls are called “hana-dama” while the inferior quality pearls are called “do-dama” and “kuzu”.
Hyriopsis Cumingii Freshwater pearl-producing mollusk of the family Unionidae. It lives in the waters of China and is also known as “Triangle shell mussel”. In Chinese it is called “San-Jiao-Bang”. It has been recently used in the Chinese cultivations of Freshwater pearls to obtain non-nucleated cultured pearls of a better quality (round and smooth) and bigger dimensions (up to 12 mm in diameter) in relation to the ones obtained by the Cristaria-Plicata or “Cockscomb pearl mussel”.
Hyriopsis Schlegeli Freshwater pearl-producing mollusk of the family Unionidae, also known as “Ike-Cho-Gai” and “Biwa pearly mussel”. It is used in Japan and China for the production of Freshwater cultured pearls. The oval shell is very resistant and can reach 20-30 cm in length. In 1924 it was used by Kokichi Mikimoto in the Biwa and Kasumigaura lakes to produce the first non-nucleated cultured pearls because it was impossible to graft a rigid nucleus of mother-of-pearl in its twisted visceral mass.
Hyriopsis-Schlegeli Cumingii Hybrid pearl-producing mollusk (Hyriopsis-Schlegeli and Hyriopsis-Cumingii) that is very resistant and productive. It is absolutely the most used mollusk in the cultivations of Freshwater cultured pearls in China and Japan.
Hybrid Cross-breed between two different species of the same genus.
Half pearls Incomplete pearls grown against the inside shell. They don’t have nacreous layers on the base.
High luster Highest brightness of a pearl, observed in a circumscribed and well-defined area of the pearl.
Ike-Cho-Gai See Hyriopsis-Schlegeli.
Imitation pearls See Shell pearls.
Inclusion Crystal or solid, liquid, gaseous state or multiphase material included in a gem or mineral.
Infiltration Treatment done on non-nucleated cultured pearls to stick the nacre detached from the nucleus of mother-of-pearl.
Infrared Electromagnetic radiation with a wavelength longer than that of visible light. Such radiations have a wavelength interval between 700 nm and 1 mm.
Interference Optical effect caused by the influence and overlapping of two homogeneous light waves with the same frequency and approximately the same directions.
Iridescence Play of infinite multicolor reflexes that are visible over the surface of a pearl after a light interference on the outer layers of the nacre. We can see that when there is the phenomenon called orient.
Irradiation Treatment done on cultured pearls in order to modify their color through atomic or subatomic radiations.
Inner epithelium Covering tissue of the mantle of mollusks. It is formed by epithelial cells that constitute the breathing system of the organism.
Improved cultured pearls See Shell pearls
Irregular pearls Pearls with and irregular, undefined surface. The term is a synonym for baroque pearls.
Jomon 5.500 year-old Japanese Akoya pearl. It is considered to be the oldest natural pearl of the world.
Japanese pearls Term used to indicate the Akoya pearls, which are cultured pearls produced almost completely in Japan.
Kan Ancient Japanese unit of weight used to weigh pearls. It corresponds to 1000 momme (= 3,75 kg).
Karasu See Hyriopsis-Schlegeli.
Kasumiga Freshwater Japanese cultured pearls. They are round and have their rigid nucleus derived from low quality Akoya pearls.
Kasumigaura See Hyriopsis-Schlegeli.
Keshi-perle Saltwater or freshwater non-nucleated pearls generated from a cultivation incident. They don’t have a nucleus and present irregular forms of small dimensions.
Kobe-perle See Shell pearls.
Kapis Shell (Windowpane oyster) See Placuna Placenta.
Laue-gram Printed image on a plate by the x-ray diffraction through aragonite crystals that compose pearls. Natural pearl Laue-grams are senary. Those of cultured pearls are senary and binary.
Linné Carl von Linné was a Swedish naturalist who, in 1761, obtained for the first time cultured pearls from freshwater oysters. The used method has never been revealed but it is thought that the scientist introduced strange bodies in the mollusk through a hole on the shell.
Luster Quantity and quality of light reflected from the surface or just under the surface of a natural or cultured pearl. The higher the luster the higher the value of the pearl.
Mabe pearls Hemispherical cultured pearls that grow against the inside shell and filled with artificial material. They are also called “composite pearls“ and “half pearls“.
Mother-of-pearl Gemological material of organic or biological origins with an iridescent aspect used for decoration. It is the inner layer (also called nacreous layer) present in the valve of bivalve mollusks and in the valve of gastropod and cephalopod mollusks. It is composed of hexagonal platelets of aragonite (in a “brickwork” arrangement) bound by conchiolin. The mother-of-pearl and the nacre of pearls have the same structure and composition.
Majorca pearls Imitation pearls that consist of opalescent glass spheres coated with essence of Orient and dipped in cellulose acetate and cellulose nitrate. They are velvety and iridescent imitation pearls and are also known as “orchid” pearls.
Majorica pearls See Majorca pearls.
Mallorca pearls See Majorca pearls.
Manjari Sanskrit term that means “bud”.
Mantle Part of the anatomy of mollusks. It is the dorsal wall of the shell and covers the visceral mass of the mollusk. Its function is to intercept and block strange bodies that can eventually penetrate the shell through the slot between the valves. It is covered with a tissue called epithelium
Margaritarii Corporation of merchants of pearls (called “Margaritas”) in the ancient Rome.
Matinee necklace Uniform pearl necklace that is 50,8 – 61 cm in length. It is longer than the choker and shorter than the opera necklace.
Melo Melo Saltwater pearl-producing mollusk (common name the “Indian volute”) of the family Volitidae. It lives in the South China Sea and occasionally produces non-nacreous natural pearls known as Melo Melo pearls.
Melo Melo pearls Natural pearls occasionally produced by the sea snail Melo Melo. They are very precious pearls with an orangish-yellow color and exceptional dimensions (over 3 cm).
Mikimoto Kichimatsu Mikimoto, known as Kokichi Mikimoto, born in Japan on the 25th of January 1858. In 1913 he obtained the first round Akoya cultured pearls. From 1920 on his cultured pearls were commercialized in Europe and the United States.
Mollusk Invertebrate animal constituted mainly of a head and a body completely covered by the mantle with a significant cavity for breathing and excretion and the organization of the nervous system. The shell can be univalve or bivalve.
Momme Ancient Japanese unit of weight still used to measure export cultured pearls. 1 momme = 3,75 grams = 18,75 carats.
Nacre The same as mother-of-pearl. It constitutes the layers of natural pearls and cultured pearls.
Nautilidae Family of cephalopods to which belongs the pearl-producing mollusk Nautilus.
Nautilus Only living cephalopod with two pairs of gills. It is considered a “living fossil”. It is only found in the Indo-Pacific and occasionally produces natural pearls. It is represented by three species: Nautilus pompilius (“Chambered nautilus”), Nautilus macromphalus (“Bellybutton nautilus”) and Nautilus scrobiculatus (“Crusty nautilus”).
Nikko (perls) See Shell pearls.
Nishikawa Tokichi Nishikawa, of the Imperial Fisheries of Japan, obtained some round cultured pearls in 1907. He tried to patent his method in the same year but got it only in 1916 after his death and 50 days after Mikimoto had obtained his (Mikimoto asked for a patent of his method in 1914).
Nodipecten Subnodosus Saltwater pearl-producing mollusk of the family Pectinidae. It is known as “Giant lion’s paw”. It is native to Pacific and Gulf of California coasts of the Baja California Peninsula, Mexico, southward to the western coast of Peru. It occasionally produces natural pearls known as Scallop pearls.
Nacreous layer See mother-of-pearl.
Nucleated cultured pearls Cultured pearls produced by pearl-producing mollusks after the graft of a rigid nucleus (of mother-of-pearl) performed by a technician of the cultivation.
Non-nucleated cultured pearls Freshwater cultured pearls that don’t have a nucleus and are entirely made of layers of mother-of-pearl. They are produced by pearl-producing mollusks after the graft of an organic nucleus (a fragment of the epithelium of another mollusk). The biggest cultivations are in China (with the use of the mussels Hyriopsis-Cumingii and Cristaria-Plicata) and in Japan (initially only with the pearl-producing mollusks Hyriopsis-Schlegeli but nowadays also with a hybrid between the Hyriopsis-Schlegeli and the Hyriopsis-Cumingii).
Natural pearls Concretions mainly carbonated produced by saltwater or freshwater mollusks, with the same substances that constitute their shells. It is a biologic (or organic) product created as a defense against the accidental penetration of an irritant agent (parasites, larva, shell fragments, dead cells of the connective tissue) in the mollusk tissues through the shell.
Opera necklace Uniform pearl necklace that is 71 – 86 cm. It sits at the breastbone and can have one or more strands.
Orient Optical phenomenon that produces iridescent colors on the surface of some pearls. It is characterized by a well defined spot of light called luster that can be seen when the pearl is put in the shadow. It is caused by the diffraction and interference of light over the layers of nacre.
Organic Element with organic origins, that is, derived from living organisms (whether vegetable or animal) or related to organic chemistry (constituted of carbon, oxygen and hydrogen). Examples of organic elements are: ivory, coral and pearls.
Orchid pearls Vedi Majorca pearls.
Oyster Bivalve mollusk that has a highly calcified curved shell. It is found in a lot of seas in the world in superficial depths, near rocks or other solid bodies. Some oysters, mainly those of oriental seas, are pearl-producing.
Outer epithelium Covering tissue of the mantle of mollusks. It is composed by a layer of epithelial cells (ectoderm) and it is directly in contact with the shell. The ectoderm secretes conchiolin and calcium carbonate and it is the one responsible for the formation of the mollusk shell.
Organic nucleus Epithelial fragment, also called “graft”, introduced in a pearl-producing mollusk through the grafting operation, in order to stimulate the formation of a non-nucleated freshwater cultured pearl.
Pallial Refers to the mantle (by the Latin pallium = mantle). It’s the cavity of the body of mollusks where the gills are situated.
Paris pearls Also called “French pearls”. It refers to the imitation pearls created by the Frenchman M Jacquin in the 17th century. He is the one responsible for the creation of the “Essence of Orient” (substance made up of iridescent small crystals taken from fish scale and an organic liquid that is usually cellulose nitrate). He then took small spheres of hollowed glass, coated their inside surfaces with the pearly mixture and filled them with wax.
Pelecypod See Bivalve.
Perna Latin term that literally means “pig’s thigh”. Ancient Romans used it to refer to the shell of the mollusk Pinna Nobilis (common name “Pen shell”).
Perula Latin term used by ancient Romans to refer to the pearl, probably due to its spherical shape (sphaerula, pronounced ‘sperula’).
Pinna pearls See Pleurobema Cordatum.
Pig-Toe See Pleurobema Cordatum.
Pinna Nobilis Natural pearls obtained from the saltwater mollusk Pinna Nobilis (common name “Pen shell”, “Wing shell”, “Sea-wing”). The pearls can be reddish, orangish-yellow, brown and black (the dark ones reported from the Isle of Pines and from New Caledonia). They can be roundish, elongated, drop-shaped or pear-shaped. They have an alveolar structure (when greatly magnified, it appears to be formed of very small polygons, which are the bases of small pyramids radiating from the nucleus). They possess no orient, but are more highly crystalline than any other pearls.
Pinctada-Margaritifera Marine bivalve pearl-producing mollusk of the family Pteriidae. It varies into many sub-species: Pinctada-Margaritifera cumingi (Polynesia oyster that is 26 cm in diameter and known as black-lipped oyster. It is used for the cultivation of the Tahiti pearls); Pinctada-Margaritifera erythrensis (found in the Red Sea, Madagascar, Tanzania, Seychelles, North Australia, Ryu-Kyu islands, America) and Pinctada-Margaritifera persica (Persian Gulf oyster in Southwest Asia).
Pinctada Martensii Fucata Marine bivalve pearl-producing mollusk of the family Pteriidae, also known as Akoya-Gai. It is found in the seas of Japan where it is used for the cultivation of the Japanese Akoya pearls. It reaches 10 cm in diameter at most and has a delicate white mother-of-pearl with rosy shades.
Pinctada Mazatlanica Marine bivalve pearl-producing mollusk of the family Pteriidae. It lives along the Mexican coast (Baja California) and the Panama coast and it is also known as “Panama black-lipped mussel”. The shell, as big as 20 cm, has a green inner edge and outside it is brownish-yellow. It occasionally produces natural pearls called Cortez.
Pinctada Maxima Marine bivalve pearl-producing mollusk of the family Pteriidae, also known as silver-lipped oyster or gold-lipped oyster due to the silver or gold color of the valves. It is found in Australia, Indonesia, Philippines and Polynesia and has a huge shell (up to 30 cm in diameter and 5kg). It is used in Southeast Asia to produce the so called South Sea pearls.
Pinctada Penguin Marine bivalve pearl-producing mollusk of the family Pteriidae, also known as “penguin wing oyster”. It has a multicolored (shades from pink to red) and iridescent (bluish shininess) shell. It is used to produce the so called Mabe pearls in Amamioshima, near Okinawa, in Japan.
Pinctada Radiata Marine bivalve pearl-producing mollusk of the family Pteriidae, also known as “Atlantic pearl oyster” or “Gulf pearl oyster”. It is found in the Persian Gulf, along the Indian and Venezuelan coasts.
Pinctada vulgaris Marine bivalve pearl-producing mollusk of the family Pteriidae. It is found in the Indian Ocean, Red Sea and the Persian Gulf and its shell is up to 10 – 12 cm in diameter.
Pink pearls Calcareous concretions occasionally produced by the mollusk Strombus Gigas (common name “Queen Conch”) as a reaction of the accidental penetration of a strange body through the shell. Some scientists say they are kidney stones instead. Pink pearls can be only natural pearls, because the cultivation is not possible. They are small (2-3 mm in diameter), mass-reduced (0,8-1,2 grains = 0,2-0,3 carats) and irregular shaped (roundish baroque). The color presents the most varied shades of pink (purple, salmon, yellowish, brown, whitish) and it is caused by the presence of organic pigments called carotenoids. The surface of pink pearls is enriched by a pattern of tiny iridescent flames in matching tones.
Pinnidae Family of marine bivalve pearl-producing mollusks, to which belongs the “Pen shell” (Pinna Nobilis).
Placuna-Placenta Marine bivalve pearl-producing mollusk. It has a shell that is about 15 cm in diameter with a flat right valve and a convex left valve. The shell is extraordinarily transparent (mainly in the first years of life). That is why it is called “windowpane shell” and “Chinese glass”. It occasionally produces small and irregular natural pearls, dark and opaque of color, similar to the color of lead.
Placunidae Family of marine bivalve pearl-producing mollusks, to which belongs the “windowpane shell” (Placuna-Placenta).
Pleurobema cordatum Freshwater mollusk found in the Mississippi River, also known as Pig-toe. Its pinkish-white mother-of-pearl is used as rigid nuclei of saltwater cultured pearls.
Pearl-producing mollusk Mollusk that can produce pearls. They comprise seven classes but only three are pearl-producing species: Bivalves, Gastropods and Cephalopods.
Pteria Penguin See Pinctada Penguin.
Pteridae Family of marine bivalve pearl-producing mollusks, to which belongs the genus Pinctada.
Peeling Treatment done on pearls in order to remove the superficial layers of nacre and improve their appearance.
Pteria Sterna Saltwater pearl-producing mollusk, of the family Pteriidae, also known as “Rainbow-lipped oyster”. It is found in the Gulf of California and occasionally produces natural pearls called Cortez pearls.
Pearl-sac A cyst that is formed automatically after the penetration (graft) of a fragment of epithelium in the mantle of the oyster. If the epithelium survives there will be the formation of the pearl-sac and then of the cultured pearl.
Quadrula Genus of freshwater bivalve pearl-producing mollusks.
Queen Conch See Strombus gigas.
Roman pearls Imitation pearls. Small alabaster spheres dipped in iridescent substances.
Red pearls Imitation pearls. Small alabaster spheres dipped in iridescent substances.
Rice pearls Freshwater cultured pearls or natural pearls that have a flat oval shape of small dimensions. They are cultivated in China by the pearl-producing mollusk Cristaria-Plicata (“Cockscomb pearl mussel”).
Rigid nucleus Small sphere of mother-of-pearl derived from the shell of a donor mollusk and introduced in a pearl-producing mollusk through the grafting operation in order to stimulate the formation of saltwater cultured pearls (Akoya, South Sea and Tahiti). Rarely, instead of the mother-of-pearl, grafters use a small natural pearl or a small sphere of conchiolin and calcite dust.
Sankakuho See Hyriopsis-Schlegeli.
San Jiao Bang See Hyriopsis-Cumingii.
Sautoir Pearl necklace that is longer than the length of an opera necklace.
Scallop pearls Natural pearls occasionally produced by the saltwater mollusk Nodipecten Subnodosus (common name “Giant lion’s paw”). They are symmetric pearls of dark and deep colors (burgundy, plum, violet…).
Sea-Wing See Pinna Nobilis.
Senary Diffraction diagram in which there are six central dark spots hexagonally arranged. It is obtained by x-raying an aragonite geminate perpendicularly to the pseudo-hexagonal base. When there are two senary diagrams obtained in perpendicular directions, it means it is a natural pearl.
Sea silk See Bisso.
Sheba pearls See Shell pearls.
Shiwa-Gai See Hyriopsis-Schlegeli.
Soufflure A hollow in a baroque pearl, formed when an organic irritant decomposes inside its pearly coating before the nacre hardens. Gases released by the rotting object inflate the nacreous buildup like a bubble.
South Sea (perls) Saltwater cultured pearls, with a rigid nucleus, able to reach remarkable dimensions (diameter from 9 to 17mm). They are divided into two big groups: white pearls and black pearls. White pearls are cultivated in the Pinctada Maxima oyster (the White-lipped and the Gold-lipped oysters) in Australia and marginally in Indonesia, Philippines, Thailand, Myanmar and Okinawa. Black pearls (of which make part the legendary black Tahiti pearls), are cultivated in the Pinctada-Margaritifera oyster ( the Black-lipped oyster) in a wide zone of South Pacific. South sea pearls are the most expensive and desired ones due to their little availability.
Strombidae Family of gastropod pearl-producing mollusks, to which belongs the mollusk Strombus Gigas (common name “Queen conch”).
Strombus Gigas Saltwater pearl-producing mollusk of the family Strombidae, also known as “Queen conch”. It is found in the Caribbean seas and occasionally produces natural pearls known as Conch pearls.
Structure (of pearls) Pearls are composed by thousands of extremely thin layers of nacre, which are invisible even if put under a highly magnifying microscope. In the pearl formation, first is created a kind of thin membrane that is similar to a net, known as conchiolin, in which later are deposited thousands of tiny aragonite crystals.
Sumo pearls See Shell pearls.
Shell pearls Imitation pearls. Small man made spheres of mother-of-pearl coated with iridescent thin layers of nylon and dipped in varnish (a mixture of plastic substances, lead carbonate, mica, titanium oxide). They are inexpensive and very popular and are also called by fantasy names (Angelo, Sheba, Mikomo, Takara, Kobe, Nikko, Sumo, Fiji, Aloha) or even worse, improved cultured pearls.
Shell A hard, protective outer layer created by many bivalve and univalve mollusks (composed mainly by calcium carbonate).
Silver-lipped oyster See Pinctada Maxima.
Sea ears See Haliotis-Gigantea.
Saltwater cultured pearls Cultured pearls of oysters or other saltwater mollusks. The main varieties are the Japanese Akoya pearls, the Australian South Sea pearls and the Tahitian pearls.
Saltwater natural pearls Pearls produced spontaneously by saltwater mollusks. The most important ones belong to the families Pteriidae, Pinnidae, Tridacnidae, Placunidae, Strombidae, Haliotidae, Volitidae, Nautilidae.
Surface Quality evaluation category used to describe the amount of blemishes on the surface of a pearl or cultured pearl. Blemishes may include cracks, chips, dull spots, wrinkles, spots, holes, bumps, and pits. The presence of any of these blemishes diminishes gradually the value of the product. Surface descriptions range from clean (no visible blemishes) to heavily blemished.
Tahitian pearls Saltwater cultured pearls produced in Polynesia, also known as black pearls. The deep colors, the richness of reflexes and the extraordinary luster have made Tahiti’s pearls the most fascinating and exclusive cultured pearls in the market.
Takara pearls See Shell pearls.
Treatment Artificial modifications (chemical and/or physical) on a gemological material in order to improve its appearance. When it comes to pearls, the most common treatments are: dyeing, bleaching, peeling, waxing, infiltration, irradiation, coating, coloration.
Tridacna Gigas Bivalve saltwater pearl-producing mollusk of the family Tridacnidae. It has extraordinary dimensions (as much as 150 cm in length and can weigh more than 200kg) and is known as the “Giant clam”. In the past its valve was used as a stoup in churches. It occasionally produces whitish, milky, irregular-shaped, non iridescent pearls, known as conch.
Three-quarter pearls Incomplete pearls, grown against the inside shell. See also half pearls.
Tridacnidae Family of bivalve pearl-producing mollusks to which belongs the mollusk Tridacna Gigas.
Trigeminate Geminate composed by three crystals.
Tritigonia Freshwater bivalve pearl-producing mollusk.
Turbinidae Family of gastropod pearl-producing mollusks, to which belongs the mollusk Turbo Petholatus.
Turbo Petolatus Univalve saltwater pearl-producing mollusk of the family Turbinidae. It is found in the waters of Polynesia and occasionally produces greenish-yellow pearls.
Translucency Optical effect that makes the pearl surface semi-transparent and luminous. It derives from the presence of other two optical effects: luster and orient.
Triangle shell mussel See Hyriopsis-Cumingii.
Turkey pink pearls Iridescent purple-red natural pearls, produced by the oyster known as “Red abalone” (Haliotis-Rufescens).
Umbone Central swelling surrounded by the concentric lines that decorate the valves of the bivalves. It is also called apex.
Unio margaritifera Freshwater bivalve pearl-producing mollusk. It has a solid shell (oval and elongated) and is found in North America and Europe. It occasionally produces small pearls, with a great variety of colors (white, pinkish, gray, brown, black and mixed). Since the 80’s, in the United States, the mollusks from the genus Unio have been used for cultivations.
Unionidae Family of freshwater bivalve pearl-producing mollusks, to which belongs the mollusk Unio-margaritifera.
Univalve Mollusk with a one-valve shell (from the family of the gastropods and cephalopods).
UNI rules Rules written and published by the UNI (Italian Naturist Union) that regulates many commodity sectors. In the gemological field there are the Rules 9758 (Diamonds), 10173 (Cutting classification), 10245 (Gemological material) and 9810 (Color denominations). Pearls are treated in the Rule 10245.
Valve The two components of the shell of bivalve mollusks.
Venetian pearls Imitation pearls. Glass beads mixed with pearly substances during their fusion.
Volitidae Family of gastropod pearl-producing mollusks to which belongs the mollusk Melo Melo.
Wing-Shell See Pinna Nobilis.
Waxing Treatment done on pearls that consists of impregnating the superficial layer of the pearl with wax or vegetable oil in order to give more shininess to the pearl. It is a short duration effect.
Weave (of pearls) Term that refers to the edges of the nacre layers that intersect the surface of a pearl in infinite lines, reminding us of a finger print.
Xeroradiografia Radiography using X-rays and xerographic techniques. The essential part of the process is the xerographic plate that is made of an aluminum base coated with a thin layer of selenium. After an object has been exposed to the radiations a latent electrostatic image is formed, highlighted with an appropriate dry development.
Ye-Jing-Yang Chinese character who tried the cultivation of pearls in the 12th century, by inserting small statues of Buddha between the mantle and the inner surface of the mollusk shell. After some years, the statues would get coated in mother-of-pearl.

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