Antoinette L. Matlins

Get intimate with gems. With the loupe, examine the cut (if faceted), setting, proportion, the depth and interior.
Cuts: is the girdle or table damaged, the stars and kite facets rounded off? Common cuts: Brilliant ("diamond" shaped), trillion, pear, marquise or navette (pointed oval), oval, emerald (cornered rectangle), radiant (square), heart, baguette (bevelled rectangle), and tapered baguette.
Settings: Bezel, prong, fishtail, belcher (carve-outs), gypsy (often with cabachon, a fitted well), illusion, flat-top, channel, pave', cluster, solitaire, ballerina, etc.
Factors to determine value: body color and grade, clarity (flaws), cut and proportion (the "make" -- from culet, the point below the pavilion facets, to the table), and carat weight.
Internal flaws (inclusions--including thin flat mirrors known as dark spots, and feathers, bearding, grain lines, colorless crystals, and cleavages), and externals, or blemishes (knaat or twin lines/ ridges, laser treatments (to vaporize black spots, leaving fine white threads). Note the trigons and small naturals, usually at the girdle. Rarely, may see roughness, scratch, abraded or rough culet, and polishing lines. Minor flaws.
A flaw right under the table may be the least desirable as most visible, and most affect on brilliance and fire. It may have "reflections" -- as many as 8 times within the stone.
Slightly imperfect stones are preferred. [47. That's what they all say!] Color and brilliance are the most important factors.… (plus d'informations)