Coating, Spraying and Foiling

  • Nature of gemstone: Coating is done on almost any gem stone. Coating can be performed both on rough and cut stones.
  • Materials used: Wax, plastic, paints, resins, reflecting coloured papers and metallic foils, sputtering techniques etc. are used.
  • Possible effects:
    1. In diamond, a special coating is performed known as Carbon Vapor Deposition (CVD) where a thin layer of synthetic diamond is deposited over a colourless stone to imitate diamond. One the latest coated colours are the black and green coated diamonds. The black diamonds appear opaque black but may actually be dark browns which are semi transparent in transmitted light. The coating is probably due to a layer of diamond - like carbon (DLC).
    2. In the case of colourless topaz or quartz a thin layer of gold is deposited over the colourless topaz or quartz to produce a blue colour. This has been marketed under the trade name aqua aura. A variety of metallic elements are currently being used to produce different colours which are subsequently marketed as red aura etc.
    3. A coloured paper or metallic foil is pasted on the pavilion facets of cut gemstones or the base of the cabochon to enhance the colour as well as the quality of light reflected from the stone.
  • Identification:
    1. Detection in loose stones can be done by boiling with acids or using a hot point or by scratching the stone with a pointed fragment.
    2. In the case of mounted stones, with the help of a lens, examine the back portion of the stone through the table. Some form of crinkling or wrinkling of the coating or foiling is visible.
    3. Identification is also possible by a careful examination of the unevenness of colour on the surface.