Pearl Gemmology Review Flashcards

A set of 60 vocabulary flashcards covering pearl composition, formation, types, grading, and testing based on the Gem-A Diploma Study Guide.

Nacre

An organic-inorganic composite material forming the outer layer of a pearl and the inner layer of the shell, composed of overlapping platelets of aragonite interlayered with a thin organic conchiolin 'cement'.

Mother-of-pearl (MOP)

The nacreous inner layer of the shell, used to make bead nuclei for cultured pearls.

Conchiolin

The dark, horny organic material forming the outer shell layer and the cement between aragonite platelets in nacre.

Orient

The unique, complex lustre of pearl produced by surface reflection of light and subtle iridescence from overlapping nacre platelets.

Nacre thickness

The depth of nacre over the bead nucleus in a cultured pearl, typically ranging from $0.2–1\,mm$ for marine cultured pearls.

Mantle

The soft tissue of the mollusc in contact with the inner shell layer that secretes nacre and shell material.

Cyst pearl

A pearl forming inside the body of a bivalve when a mantle pearl sac separates from the main mantle; the familiar round pearl is an example.

Blister pearl

A pearl formed when nacre secretion between the shell and the mantle creates a dome-like protrusion attached to the inner shell.

Mabe pearl

A cultured blister pearl produced by inserting a dome-shaped nucleus between the mantle and shell of the mabe oyster.

Tissue nucleation

A culturing method using only mantle tissue without a bead nucleus, mostly used for freshwater pearls.

Bead nucleation

A culturing method using a mother-of-pearl bead nucleus and mantle tissue inserted into the host oyster.

Seed pearl

A small pearl less than $2\,mm$ in diameter, which may be natural or a by-product of culturing.

Baroque pearl

A pearl of very irregular shape.

Bivalve

A mollusc whose shell consists of two hinged parts, including oysters and freshwater mussels.

Gastropod

A mollusc with a single shell and a flattened muscular foot, such as a conch, melo snail, or abalone.

Akoya pearl

Small white marine cultured pearls from Pinctada fucata, typically beaded with a nucleus of approximately $6\,mm$.

South Sea pearl

Large white or golden cultured pearls from Pinctada maxima, with beads up to $12\,mm$.

Tahitian pearl

Black or grey iridescent cultured pearls from Pinctada margaritifera, with beads up to $12\,mm$.

Conch pearl

A rare non-nacreous pearl from the giant conch (Strombus gigas), usually pink to pinkish-orange with a flame-like surface structure.

Melo pearl

An orange non-nacreous pearl from the Melo melo marine snail, featuring a flame-like structure and susceptible to fading in sunlight.

Abalone pearl

Rare iridescent nacreous pearls from Haliotis gigantea, which are usually hollow and baroque in shape.

Body colour

The predominant base colour of the pearl, such as white, pink, cream, yellow, or black.

Overtone

One or more colours overlaying the body colour, such as pink/rose, green, blue, or silver.

Pearl grain

The unit of weight for natural pearls, where $1\,\text{pearl grain} = 0.25\,ct$.

Momme

The unit of weight for cultured pearls, where $1\,momme = 3.75\,g$.

Candling

A test for beaded cultured pearls where rotation in strong diffused transmitted light reveals the banded structure of the shell bead nucleus.

Guanine / Essence d'orient

Tiny crystals from herring fish scales used to coat imitation pearl beads in multiple lacquer layers.

Chemical composition

The components of pearl consisting of calcium carbonate, conchiolin, and water.

Structure

Organic or amorphous material with no crystal structure.

Specific Gravity (natural)

Value ranging from $2.60\text{ to }2.78$.

Specific Gravity (gastropod)

Value of approximately $2.85$.

Hardness

A value of $3\frac{1}{2}\text{ to }4$ on the Mohs scale.

Aragonite

A form of calcium carbonate that exists as overlapping, platy crystals in nacre.

Calcite

The prismatic form of calcium carbonate found in the middle layer of a bivalve shell.

Tooth test

A method to identify real nacre where a pearl feels gritty when gently rubbed against tooth enamel.

Pinctada fucata

The scientific name for the Akoya pearl oyster, which produces small white marine cultured pearls.

Pinctada maxima

The scientific name for the silver or gold lip South Sea oyster, producing pearls up to $18\,mm$.

Pinctada margaritifera

The scientific name for the black-lipped South Sea oyster found in Tahiti and the Cook Islands.

Strombus gigas

The scientific name for the giant conch, producer of pink non-nacreous pearls.

Melo melo

The scientific name for the marine snail that produces orange non-nacreous pearls.

Haliotis gigantea

The scientific name for the abalone, which produces colourful iridescent baroque pearls.

Hyriopsis schlegelii

A freshwater mussel species used in Japan for beadless cultured pearl production.

1896

The year Kokichi Mikimoto patented the blister pearl culturing process in Japan.

~1905

The approximate year round cultured pearls first appeared on the market.

Lake Biwa

The location in Japan where the first beadless cultured freshwater pearls were produced.

China (Freshwater Production)

The largest producer of freshwater cultured pearls worldwide by volume.

Bleaching

A common trade practice involving soaking pearls in agents like dilute hydrogen peroxide to lighten colour.

Silver nitrate treatment

A staining method where silver deposits create a permanent grey or black colour in pearls.

Irradiation

A treatment that turns freshwater cultured pearls and their freshwater bead nuclei grey.

Surface condition

A quality grading factor involving the assessment of blemishes such as spotting, pits, welts, and circular markings.

Lustre

The most important quality factor, defined as the brilliance and mirror-like quality of the pearl's orient.

Hope Pearl

A famous irregular pear-shaped baroque pearl weighing $1800\,grains$ ($450\,ct$).

Periostracum

The thin, dark horny organic outer layer of a bivalve shell, composed of conchiolin.

MOP bead source

Shells from North American freshwater mussels used to create nuclei for beaded cultured pearls.

Gonad

The part of the host oyster where the bead nucleus and mantle tissue are inserted during the marine culturing process.

Standard size measurement

The diameter of a pearl measured in $mm$ to the nearest $\frac{1}{2}\,mm$.

June

The month for which pearl is one of the designated birthstones.

Pearl sac

A structure formed from mantle cells that secretes organic material and aragonite to build a cyst pearl.

Polishing

A treatment performed on both natural and cultured pearls to improve their appearance and lustre.

Isotropic

The optical property of pearls indicating they have no measurable refractive index or double refraction.