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Heat Treatments on Rocks, Minerals, and Gemstones
Why are Rocks and Minerals Heated?
Rocks, minerals, and gemstones are heated to change or improve their colour, clarity, or physical properties. This practice can be traced back thousands of years.
As early as 72,000 years ago, and possibly as far back as 164,000 years ago in South Africa, fire was used to heat stone to alter its properties. The purpose was to improve the quality and efficiency of tools being produced.
With the passage of time, early humans continued to learn about the effects of fire on stone. They would have been aware from quite early on that when heated, some stones also changed colour.
Today, the application of heat to alter the colour or physical properties of a rock, mineral, or gemstone is known as a heat treatment.
The ancient Greek philosopher Theophrastus (c.371 - c.287 BC) wrote in depth about this in his treatise "Theophrastus on Stones".
He wrote, "Some can be burnt whilst others can be melted and then there are those which just break up into smaller pieces."
He also described the effects of moisture on rocks and minerals, saying, "Depending on texture some react differently when wet and dry." He noted that volcanic glass (obsidian) became porous when burnt and that its colour and density also changed.
He also noted that amethyst and many other stones changed colour.
Theophrastus went on to classify stones according to how they reacted to heat.
Some three hundred years later, Pliny the Elder, Roman author, naturalist and philosopher, also noted the effects that heat had on stones. He noted, "one gemstone could be changed into another" and "the colour of crystal quartz could be changed into that of an emerald."
These statements show that he was aware that heat could alter the colour of stones.
In chapter 75 of his encyclopaedia, Naturalis Historia, Pliny writes;
There is considerable difficulty in distinguishing genuine stones from false; the more so, as there has been discovered a method of transforming genuine stones of one kind into false stones of another.
Sardonyx, for example, is imitated by cementing together three other precious stones, in such a way that no skill can detect the fraud; a black stone being used for the purpose, a white stone, and one of a vermilion colour, each of them, in its own way, a stone of high repute.
Nay, even more than this, there are books in existence, the authors of which I forbear to name, which give instructions how to stain crystal in such a way as to imitate smaragdus and other transparent stones, how to make sardonyx of sarda, and other gems in a similar manner. Indeed, there is no kind of fraud practised, by which larger profits are made.
Ref: (Pliny's Naturalis Historia chapter 75) Please Note.
Smaragdus is an ancient name for emerald. Sardonyx and sard are both types of carnelian.
Heat-Treated Stones Today
Different heat treatments are used to make subtle improvements to the colour of minerals and gemstones or to change their colour completely.
Although blue topaz occurs naturally, it's extremely rare. Most blue topaz is produced by irradiating colourless topaz, followed by heating.
Irradiation changes a gemstone’s colour by altering its internal structure at an atomic level. Conventional heating works differently, relying on high temperatures to cause chemical or structural changes. In some gemstones, irradiation creates the colour and heat is then used to stabilise it. This is why different or multiple treatments are often used.
Tourmaline, which can be quite dark, is often heated to lighten the colour.
Amethyst is often heated to produce citrine. Greener shades of aquamarine can be heated to turn the stone blue. Most of the world's finest rubies and sapphires are heated to improve colour, clarity or both. It's extremely unusual for these gemstones not to be heated.
Most tanzanite is heated to change or enhance the depth of colour. Heating also removes undesirable yellow or brown tints.
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Sardonyx: Courtesy of Captain Tenneal. Sard: Stone Mania.
