What is Achat? 
Achat is a German term for agate.

What is Acetylene Tetrabromide? 
​Acetylene tetrabromide is  C2H2Br4 S.G. 2.964 (at 20° C.) which can be lowered by mixing with alcohol (Smith). Is lowered by dilution with toluol (R. Webster). A heavy liquid.

What is Acentela? 
Acentela is a spanish term for  Rock crystal.

What is an Accidental Pearl? 
Accidental pearl. Genuine natural pearl as distinguished from (artificially induced) cultured pearl. A term not used in the trade as it is of questionable meaning.

What is Accarbaar? 
Accarbaar is a southeastern Asiatic name for black coral.

What is Absorption?
absorption. (a) White light is a combination (blending) of those hues of the spectrum which are seen in the rainbow. The hue of a gemstone is due to the absorption of certain portions of white light in its passage through the gemstone. The remainder of the light which is not absorbed in the gemstone combines or blends to produce the hue seen. This process is called selective absorption. See also selective reflection.
(b) The exact portions of white light which are absorbed by a gemstone or other substance may be determined by means of the spectroscope forming a band of colors known as an absorption spectrum.
(c) Dark zones crossing the spectrum represent the portions of the light absorbed and are known as absorption bands or absorption lines.

​What is Abrasive? 
Abrasive (ab-rae′siv). A substance such as emery (powdered corundum) used to wear away another substance by friction. Carborundum, diamond powder and other abrasives are used in fashioning gemstones.

What is abrade? 
​Abrade is to wear away by friction; to produce abrasion.

What is Aberration? 
Aberration (ab″er-ae′shun) is  the failure of a lens or mirror to bring the light rays to the same focus. When aberration is due to the form of the lens or mirror it is called spherical aberration. When due to the different refrangibility of light of different colors, it is called chromatic aberration. When present in magnifiers it often causes inaccurate decisions as to flawlessness or color of gems.

What is Abás?
Abás is a  Persian weight for pearls. About 2.66 troy grains.