Coating
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Definition
The application of a thin surface layer of paint, lacquer, or metallic film to a gemstone's pavilion or girdle to alter or enhance its apparent color.
Detailed Explanation
Coated gems range from obvious foil-backed stones in antique jewelry to sophisticated interference coatings applied to mystic topaz, producing iridescent rainbow colors. A classic deception is painting the pavilion of a pale diamond blue to counteract yellowish body color. Modern coatings on topaz and quartz use vacuum deposition of titanium or other metals, creating dramatic iridescent effects marketed under trade names. Coating is considered a non-permanent treatment requiring disclosure; cleaning with solvents or steam can damage or remove coatings. Detection is usually straightforward — a gentle touch with a solvent-moistened swab may remove paint.