Heat treatments have been used on rocks and minerals for many thousands of years and it's long been known that ancient man was a master of fire. 72,000 years but possibly as long as 164,000 years ago in South Africa, fire was being used in a controlled environment to heat stone in order to change its properties. The purpose of doing this was to improve the quality and efficiency of stone tools that were being produced. The application of heat with the intention of changing any of the properties within crystals, rocks and minerals is known as heat treatment.
With the passing of time man continued to learn about the effects that fire had on rocks and minerals and would have been well aware pretty early on that some changed colour as a result of this process.
The Ancient Greek philosopher Theophrastus (c. 371 - c. 287 BC) successor to Aristotle wrote in great depth in his treatise "Theophrastus on Stones" about the effects that fire had 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 that moisture had on rocks saying that depending on their texture, some react differently when wet and dry. He noted that obsidian became porous when burnt and that its colour and density also changed and that amethyst, sapphire and emerald lost colour following an application of heat. Pliny the Elder (23 - 79 AD) Roman author, naturalist and natural philosopher also wrote about the effects that heat had on rocks and minerals saying that one gemstone could be changed into another and the colour of crystal quartz could be changed into that of an emerald. Both statements make it clear that he was describing the effects that heat treatments had on certain rocks and minerals.
A variety of different heat treatments are used to both enhance and change the colour of stones and whilst the results in some may be relatively subtle, in others it can be quite dramatic. Blue topaz rarely occurs in nature and the vast majority is produced through heating clear topaz using a variety of different treatments, the result is three different shades of blue depending on the treatment that was used. Amethyst is often heated to create citrine and the banded variety is used to create prasiolite because both are relatively rare in their natural form. The greener varieties of aquamarine undergo heat treatments which transform them into a lighter shade of blue and tourmaline which is often very dark, becomes lighter following the application of heat. Some people are surprised to learn that many of the world's finest rubies and sapphires are heat treated to improve their colour and those which have not been heated are considered to be extremely unusual. Tanzanite which is extremely rare and highly sought after and comes exclusively from Tanzania is heated to enhance its depth of colour and to remove any undesirable hues of brown and yellow from the stone.
Whilst some may consider heat treatments to be cheating, the ultimate aim is to maximise the beauty of individual crystals, rocks and minerals which subsequently makes them more desirable hence increases their value. What's important to note is that heat treatments are a relatively natural process which may be likened to man mimicking Mother Nature but these treatments should not be confused with dyeing which is a totally artificial method of changing the colour of a stone.