The Gemmological Association of Australia outlines a few of the many methods used to treat quartz.
Quartz is one of the most abundant minerals and includes many well known gems which are classified as either crystalline (mostly transparent) or cryptocrystalline (mostly translucent or opaque). Gems such as amethyst, citrine, ametrine and smokey quartz fit in the crystalline group while commonly encountered stones such as onyx, carnelian, tiger eye, agate and chrysoprase are found in the cryptocrystalline group.
As quartz is such a vast group of gems, only a few of the more common treatments will be discussed here. More than likely because of their relatively low cost compared to other gemstone, many quartz treatments (and quartz synthetics) go undetected and undisclosed.
Heating is a stable, and the most common, treatment method used for the crystalline varieties. Amethyst heated to between 400º-500ºC will start turning yellow and become citrine. Naturally occurring citrine is actually quite rare and most citrine quartz sold these days is actually heat-treated amethyst. Citrine may also be the result of heat treating suitable material of brown or smokey quartz at 200ºC for one hour. Citrine that is the result of heat treatment may not exhibit pleochroism and be detected with a dichroscope.
Some ametrines (which have the appearance of half amethyst/half citrine) are also thought to be the result of heating. If amethyst continues to be heated to 575ºC it will become colourless and can develop the ‘schiller’, or appearance of moonstone (and as such is sometimes sold as a moonstone imitant). Green transparent quartz, known as prasiolite, is also the result of heat treating amethyst.
Quartz containing iron that is treated with gamma irradiation may produce amethyst colours; clear quartz under the same treatment may produce smokey quartz. Gamma irradiation followed by heating is also used to produce prasiolite.
Dyeing is the most common treatment method used on cryptocrystalline (mostly translucent to opaque) varieties of quartz. Methods include using organic dyes, inorganic compounds, or the use of wax, paraffin or plastic to fill the surface of the stone and alter colour.
Many gems purporting to be natural onyx are treated by dyeing. White chalcedony may be dyed to imitate chrysoprase and natural chrysoprase may sometimes be treated with chromium salts to enhance colour. These are just a couple of examples of two quartz varieties that may be treated. The actual list of all known quartz varieties treated by dyeing is far too extensive to be discussed here.
Detection of dyeing or filling may be carried out by using a 10x loupe and looking for concentrations of colour. A small swab (on an inconspicuous part of the gem) with acetone may wipe away some dyes. A hot point test may also be used to detect waxing or filling with another material.
So-called ‘Mystic’ quartz is a commercial name for a newer patented variety of treated quartz with a distinctive iridescent appearance. Colourless, cut and polished quartz is coated with an extremely thin titanium film that bonds with the quartz at the molecular level. The coating is permanent, however, the natural colour may be exposed with facet wear or chipping.
Readers should be aware that the above is far from a comprehensive list of known quartz treatments. For information on further reading or courses, please consult your local GAA division.
For more information contact the Gemmological Association of Australia at www.gem.org.au
Image courtesy of Terry Coldham from Sapphex.