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Turning Up the Heat
At some point, someone involved in gemstones figured out that they could throw a stone into a hot fire for a while, and it would usually come out looking better than when it went in. Whether it was by accident, or if someone was just playing with fire, no one knows. But the fact was established: heat treatment had been discovered and has been employed to enhance the color and clarity of many, many kinds of gemstones ever since.

While that may be an oversimplification, the fact is that heat treatment can lighten overly dark material, make pale material more vivid, or reduce a color component completely. It can even improve clarity and heal internal fractures in some species. The whole purpose is to improve the supply of desirable material; since the color and clarity of many gem species can be improved, it makes some material that was potentially unsalable now salable.

Legitimate documentation goes back to Pliny the Elder, who noted that the color of a gemstone could be altered by heating it in a crucible and using a blowpipe to intensify the heat. But the actual practice probably dates back a couple thousand years earlier to Ancient India or Ancient Mesopotamia, where specimens of carnelian show evidence of being heated (and of other treatments). The heat treatment techniques used in Thailand and Cambodia have not changed significantly for hundreds, if not thousands of years.

Equal Opportunity
For thousands of years, fine gems were only available to the societal elite (ironically, it’s impossible whether or not to determine if these fine gems had been subjected to heat treatment, given the age of the practice). When heat treatment gained broad recognition and acceptance in the early 1900s, it made gems of apparent top-notch quality more affordable to the masses without resorting to synthetic gemstones (which had been available since the 1880s).

And therein is the biggest difference. Gems that are heated are almost always less valuable than untreated gemstones that are naturally fine in quality. The price difference can vary by species, from a few dollars per carat in quartz, to tens of thousands of dollars per carat in ruby. So the truly rare and superlative gemstones of the world are still reserved for those of a mind to obtain them.

To be honest, the growth of the jewelry industry wouldn’t have been half of what it was during the 20th Century were it not for technological advances that streamlined the heat treatment process. Scads of marginal ruby, sapphire, tourmaline, topaz, quartz, zircon, and beryl would be lying useless in a heap were it not for heat treatment. Tanzanite, one of the world’s most popular gemstones, would be virtually nonexistent, and certainly not of sufficient supply to be enjoyed in jewelry with any regularity.

Know Your Stuff
Aside from the fact that it exists, the big thing to know about heat treatment is that it is a permanent, stable treatment. Some gem enhancement techniques are only temporary or can be jeopardized by normal wearing and cleaning routines. For the most part, heating is only alterable under conditions that are typically only encountered when it is done intentionally. Unless you happen to frequent a steel mill while wearing your goods, the chances of your jewelry encountering sufficient heat for sufficient time to affect its treatment are next to nothing.
Images courtesy of Gem Mountain Sapphire Mine
All of the gemstones listed below are known to be beneficially affected by heat treatment. If you are shopping for jewelry featuring, or loose specimens of, any of these stones, you should assume that everything you see has been heat treated unless specifically told otherwise on individual examples.
Ruby (corundum)
Sapphire (corundum)
Topaz
Zircon
Tanzanite (zoisite)
Carnelian (chalcedony)
Amber
Amethyst (quartz)
Citrine (quartz)
Smoky Quartz (quartz)
Ametrine (quartz)
Prasiolite (“Green Amethyst” – quartz)
Aquamarine (beryl)
Morganite (beryl)
Heliodor (beryl)
Kunzite (spodumene)
Others to keep an eye out for are spinel and some demantoid garnet, all of which are more recently known to have undergone heat treatment from newer mines. It is still safe to assume that stones from historic sources are untreated. Red, pink, orange, and yellow tourmalines are all known to be regularly heated, or otherwise treated, as well.

Finding Unheated Material
If you’re specifically looking for gems that have not been heated, there are species that are rarely, if ever, heated. Emeralds, for instance, are never heated if only because they wouldn’t survive the treatment. Garnets (apart from demantoid) are almost always completely untreated, as is peridot. Lesser known gems like andalusite, iolite, and kornerupine can also largely be assumed to be untreated.

If you’re looking for one on the list above, certified unheated material is available, but we’ll need to track it down for you. Unless an individual stone’s origins can be traced back to the mine, hand by hand, it can be difficult, if not impossible to ascertain that it has not been heated after mining.

The main reason for this is that some evidence gemologists look for of heat treatment can be brought about by natural processes. Low heating to improve color is virtually indistinguishable from the effects of natural heating while a stone is in the ground. Even internal evidence, like tensions haloes and shattered crystal inclusions, can be caused by natural occurrences.

Due to this, some labs won’t give a definitive answer on treatment status of a stone you send to them for investigation. For example, the GIA, in some instances, will state “No treatment detected” when offering an opinion on a stone’s treatment status. That is not the same as “Non-treated.”

Treatment status of gemstones is, in many countries, a mandatory disclosure from buyer to seller. With the potential ambiguity of heating, most stores will simply say as we have: unless we can definitively tell you otherwise, assume it is heated.