Off-color diamond: Any diamond that has a tinge of undesirable color, especially yellowish, brownish or grayish.
Old-mine cut: An early form of a brilliant-cut with a nearly square or cushion-shape outline. Not to be confused with European-cut.
Radiation treatment exposes off-color diamonds or slightly tinted stones to certain types of radiation to create a fake fancy-colored diamond.
Perhaps the most nefarious of the fraudulent uses of coating is an old (but still in use) trick of putting a tiny drop of indelible blue ink or paint underneath the prongs holding an off-color diamond in its setting.
 See also: Regard, Blemish, Surface, Colorless, Colored stone
  
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