Gemstone Terminology

63 Essentials You Should Know by Sharon Elaine Thompson

If you've just started making jewelry or just begun adding gem materials to your work, you may find you're being bombarded with a slew of new and unfamiliar terms. This short glossary is a basic intro to some common terms from the fields of gemology (the science of gems) and lapidary (the art of cutting gems) that will help to get you in the know.

Adularescence is a floating bluish light also known as "the moonstone effect/7

Photo Gemological Institute of America. Reprinted by permission.

Adularescence is a floating bluish light also known as "the moonstone effect/7

Photo Gemological Institute of America. Reprinted by permission.

Adularescence (or "the moonstone effect") The floating bluish light seen in the best quality moonstones; a phenomenon caused by the interference of light from microscopically thin layers of the two types of intergrown feldspar (orthoclase and albite) that make up moonstone. Some of the effect may be caused by minute particles in the stone scattering the light.

Asterisir A phenomenon called "the star effect"; most well known in rubies, sapphires, and black sapphires. Caused by light reflecting off of needle inclusions that have grown at angles to one another.

The angles are determined by the mineral-crystal system in which the stone forms. Corundums (rubies and sapphires) form in the hexagonal system; they have three axes and six-legged stars. Garnets form in the cubic system; they have two axes and four-legged stars.

Before any stone is cut, the light reflecting from its inclusions appears as a silky sheen. In order for the reflecting light to form a star, the stone must be cut into a cabochon; the dome of the cab allows the light to reflect off the surface of the needles.

Aventurescence (or "schiller") A spangled, glittery phenomenon caused by light reflecting off of flat crystals inside a gemstone; most commonly seen in aventurine quartz and sunstone feldspar. The color of the aventurescence depends on the color of the stone's inclusions.

An imperfection, such as a scratch, a chip, or a fracture, on the surface of a stone.

Body coioi The dominant hue within a gemstone. The body color is best determined by viewing the stone table-side down under a light. See also key color.

Brilliance The amount of light, specifically white light, returned from a gemstone to the eye of the viewer; a combination of reflected surface light and light reflected from internal surfaces in transparent stones.

Cabocho; A gemstone-cutting style in which the bottom of the stone is flat and the top is domed. Although transparent stones may be cut into cabs, the style is most often used for translucent or opaque stones, or to magnify or exhibit certain types of phenomena, such as asterism or chatoyance.

earner A gemstone-cutting style in which a design or image is raised above the surface of the stone. The most common cameos are made of shell. Usually the color of the image is in contrast to the background.

Cat's-eye effect See chatoyance.

Chatoyance The "cat's-eye effect." A phenomenon caused by light reflecting off of needle inclusions. Unlike in asterism (the star effect), in the cat's-eye, needles are oriented in only one direction.

Before any stone is cut, the light reflecting from its inclusions appears as a silky sheen. To form a cat's-eye, the stone must be cut into a cahochon.

Cleavage The tendency for a gem material to break between planes of atoms, resulting in a flat, usually highly reflective surface.

Some gem materials, such as topaz and kunzite, cleave along these planes more readily than others, making the stones fragile or brittle. Be careful while setting and wearing such stones.

"Cleavage" is also the name given to an inclusion caused by this kind of break when the break remains entirely within the stone.

Aventurescence (or "schiller") is commonly seen in sunstone feldspar. Gem courtesy of

The same alexandrite exhibits different colors in daylight, top, and incandescent light, bottom, demonstrating "color change."

Photos £> Gemological Institute of America. Reprinted by permission.

The same alexandrite exhibits different colors in daylight, top, and incandescent light, bottom, demonstrating "color change."

Photos £> Gemological Institute of America. Reprinted by permission.

Color change (or "the alexandrite effect") A phenomenon that causes gemstones to appear different colors depending on whether they are seen in fluorescent or incandescent light; caused by the interaction of light with the atomic structure of the stone. Most pronounced in alexandrite chrysoberyis; stones that exhibit color change are often described as alexandrite-like.

Conchoidal fracture A curved or shell-like break within or on the surface of a stone.

Crown The top of a faceted gem-stone; the area above the girdle. (See "Anatomy of a Gemstone," opposite, top.)

Culet The small flat facet on the apex of a faceted stone's pavilion. (See "Anatomy of a Gemstone," opposite, top.) A culet helps prevent the chipping and breakage that can result if two flat facets meet at a sharp angle.

Dichroism The tendency for some doubly refractive stones to exhibit different body colors when seen from different directions. Iolite is a dichroic stone.

Diffusion treatment Heating a cut gemstone (often a corundum, such as ruby or sapphire) at high temperatures in the presence of chemicals so that the chemicals are absorbed into the surface of the stone, usually altering its color.

Because the coloration is shallow, it can be removed during recut-ting and will be revealed if the stone is chipped or broken. All treatments should be disclosed to the buyer.

Dispersion The rainbow effect that's visible when white light goes through a prism, bending and breaking into the colors of the spectrum. Occurs in strongly refractive gemstones, such as diamond. The amount of dispersion that's visible is affected by the angles of the cuts.

Double refraction The ability of a gem material to break a single light ray into two when the light enters the stone.

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