Common Gemstone Treatments

Tourmaline, often occurs in very dark shades of green, so dark as to appear almost black. This as well as the dark blue material can sometimes be made lighter (and the green possibly more greenish) by heat treatment. The temperature must be controlled carefully as overheating may cause destruction through loss of water. Such heat treated materials tend to be somewhat more brittle than untreated material, which may show itself in a tendency to abrade along facet junctions. Sometimes heat treatment of dark green tourmaline results in a structural alteration at the surface; this is believed to be the cause of so-called satellite reading noted when a refractive index is taken on such material.

Zircon. Heating to temperatures in the range of 900° - 1000°C is used to produce the commercially most important colors of zircon. Reddish brown stones are first heated in a reducing atmosphere, which may alter the color of the stones to blue, colorless, or an undesirable off-color. Those that have not turned blue or colorless may next be heated in oxidizing environment, converting some to colorless and others to a yellow, orange or red color. Stones that still have not taken on a marketable color may be heated further in either atmosphere, and some stones may go through several heating. While virtually all of the heat treated may be quite stable to light and reasonably high temperatures, some heat treated zircons will revert to their original pre-treated color over time. As a precaution, such treated zircons are sometimes exposed to sunlight for several days or stored in the dark for as long as a year in an attempt to weed out unstable stones.

Tanzanite, the important gem variety of zoisite, is one of the most strongly pleochroic gems. Most of the material as mined exhibits three pleochroic colors: violet to purple, blue, and yellow to green. The third of these colors gives the stones a rather unattractive muddy appearance. Heating to relatively low temperatures bleaches the unwanted yellow to green pleochroic component, leaving the desirable violet and blue colors. Proper magnifications may reveal evidence of heat treatment, although it is usually assumed that the material in the gem market has been heat treated.