• Adamantine - resembling a diamond in hardness or luster.

  • Adularescence - an optical phenomenon applied to feldspar (usually, orthoclase moonstone, or adularia) that exhibits a floating, billowy, white or bluish light effect in certain directions as the stone is turned.  It is caused by diffused light reflection from parallel inter growths of albite feldspar, which has a slightly different refractive index from the main mass or orthoclase.

  • Alloy - the mixing of metals in set proportions.

  • Amorphous - without form.

  • Artificial Stone - a man-made, imitation or synthetic gemstone.

  • Assay - the testing of gold, platinum or silver to determine the purity of the metal.  This test is used to determine how much precious metal is contained within an item (such as a piece of jewelry) as well as other types of metal which are added to give the item its strength.

  • Assembled Stone - a gem constructed (see artificial stone) of two or more parts.  Includes foil-backed gemstones.

  • Asterism - the appearance of a rayed figure or star in a gemstone.  This is caused by light reflected from minute inclusions within the gemstone.