Common Gemstone Treatments

A number of gem materials are treated to alter their appearance. Temperatures may vary considerably depending on the material and the desired effect. In some cases relatively low temperatures may cause the desired change (s).

Amber may be heated to produce a number of desired changes. Most commonly it is heated to induce discoid fractures known as ‘sun spangles’ that some consider desirable. It may also be heated to change yellow material to a darker, more orange brown color, thereby resembling naturally age-oxidized material. Cloudy amber containing a myriad of tiny gas bubbles may be clarified by heating, usually while immersed in an oil medium (rapeseed or linseed oil). The presence of sun spangles is generally assumed to imply treatment whereas the heat oxidation and clarification processes are not normally detectable.

Aquamarine, a beryl variety, most commonly occurs in nature as a blue-green stone, the color from which it derives its name (meaning seawater). Today, however, a purer blue is more marketable. Heat treatment is therefore used to remove the yellow component of its color, leaving the stable blue. The same treatment is used to remove the yellow component from some peachy morganite beryl, resulting in a purer pink color. Because of the relatively low temperatures used, heat treated aquamarine and morganite rarely show the type of heat-treated damage used to determine heat treatment in some corundum, so detection is generally not possible.

One of the commercially more significant varieties of quartz is amethyst. The stone is heated to improve the color of dark-toned material by bleaching out some of the purple color and/or removing a smoky component. In both cases the heating (partially or completely) reverses radiation-induced structural damage that produced color centers. Heating to high temperatures is routinely used on amethyst from some sources to produce the yellow quartz variety, citrine. It is believed that practically all of the citrine on the market today is the result of such treatments. Some amethyst, upon heating, will convert to citrine in some zones and not others; this results in the bi-colored amethyst-citrine variety that has marketed under the trade name Ametrine.

Some brown to orange topaz owes its color to a combination of a chromium-produced pink component and color-center-produced yellow-to-brown component. Heating such materials repairs the structural damage that produces the yellowish component, leaving only the stable pink color. This process, which requires relatively low temperatures, is referred to as ‘pinking’. The material reportedly shows stronger dichroism than untreated natural pink topaz. Heating is also a step in the production of some irradiated blue topaz, as some irradiation sources will produce color centers for both blue and yellow to brown colors. The heating removes the yellowish component, leaving the more stable blue color.