Colorless Coatings And Impregnations

A number of essentially colorless substances have been used to coat the surface of gemstones; the primary purpose being to improve the apparent polish by masking small scratches, a grainy texture, or other surface irregularities. Coatings are also used to protect underlying dye treatments. Such treatments are primarily used on aggregate gem materials and those consisting of more than one material.

Aggregates like jadeite and nephrite have a tendency to have small crystals pulled from their surfaces during polishing. Other gem materials like lapis lazuli may show similar characteristics because they contain minerals of different hardness, resulting in the stone not polishing to an even surface. Other gem materials that may be coated include amazonite feldspar, alabaster, marble, rhodochrosite, serpentine, soapstone, and turquoise.

Among the most commonly used colorless coating materials are waxes and paraffin. Under magnification the application of a needle probe may reveal wax by removing some of it, while the application of a heated needle may cause the coating to liquefy and flow. Drill holes on beads are a good area to check as such surface coatings tend to concentrate in these.

Plastics are a class of somewhat more durable substances used to surface-coat gem materials. These too may be detected under magnification by the ability to scratch them with a needle probe. A needle may also be used to remove a small amount of a plastic coating, which then might be heated to reveal a characteristic acrid odor. A relatively thick plastic coating might be detected on the refractometer, where a reading for the plastic, or both the plastic and underlying gem material, could be seen.

Colorless impregnations are used on gem materials for a number of reasons. They are used to stabilize porous stones like turquoise by preventing skin oils from producing undesirable color changes. Some materials, because of their porous surface have a whitish chalky appearance due to the scattering effects these surface have on incident light; this can be minimized by a colorless impregnation. Such treatment is used on low quality turquoise, producing an improvement in color; it is also used on some porous, chalky opal from Brazil that does not reveal its play of color without such treatment.