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TGT Glossary of Gemological Terms

Below is a listing of terms which we would feel comfortable using on the site.  If there is something we have overlooked or for any questions, please send us an email.

Acicular:  Needle-like, as in certain types of inclusions.

Adularescence:  A cloudlike effect which moves gently over the surface of some gems.

Asterism:  The property which is responsible for the intersecting chatoyant bands that result in the star, hence the prefix 'star' for these stones.

Aventurescence:  A glitter-like effect caused by a grouping of small reflective inclusions in a stone.  Sometimes referred to as 'schiller'.

Blemish:  A surface characteristic of the stone, such as a scratch or small nick, usually the result of rough handling.

Brilliance:  The appearance of brightness that is the sum of all individual reflections of light from a stone.

Carat:  The standard unit of gemstone weight equal to 0.2 gr.  Comprised of 100 points.

Chatoyancy:  The appearance of a band of light across a stone's surface as a reflection.  These stones are known as cat's-eyes.

Dispersion:  The spectral colors resulting from the splitting of white light.

Durability:  This property refers to a gem material's ability to hold up to heat, chemicals and general wear.

Extinction:  In a faceted gemstones, areas of darkness which make the stone's overall bodycolor appear darker.  This is a product of faceting.

Facet:  The polished surface of a gemstone.

Fire:  Colored Flashes seen in a finished diamond.

Hardness:  A measure of a gem's ability to resist scratching.  This is the value indicated by the Moh's scale which ranks gem materials from 1 - 10.

Inclusion:  An irregularity in a gemstone, either completely enclosed as in a crystal, or reaching the surface as in a fracture.

Iridescence:  Similar to the colorful effect of an oil slick, this phenomena involves the appearance of spectral colors on a gem's surface.

Luster:  Degrees of luster serve as a measure of a gem's surface appearance in reflected light.

Play of Color:  The phenomena which breaks up light and gives opal its striking effect.

Pleochroism:  A property of double refractive colored stones in which varying colors are sometimes visible at different viewing angles.

Toughness:  A measure of a gem's ability to resist chipping and breaking.

Transparent:  Most typical, faceted gem materials are transparent.  They allows not only light to pass, but have the crystal structure which permits allow you to essentially see through the material.

Translucent:  A gem material which allows only light to pass (not clear images) is said to be translucent.  This term often refers to aggregate materials, jade for example, which are typically cut as cabachons.

Window:  As in extinction, this is a negative result of improper cutting in which light passes through the gem without returning to the eye of viewer.  These areas often appear washed out.