Gemstones Have Been Heated for Thousands of Years
Heat treatments have been used to enhance or change the colour of gemstones for thousands of years. It's well known that ancient man was a master of fire, 72,000 years ago and possibly as long as 164,000 years ago in South Africa fire was being used to heat stone in order to change its properties. The purpose for doing this was to improve the quality and efficiency of stone tools that were being produced. The application of heat to change the chemical composition of a rock mineral or gemstone is known as a heat treatment.
With the passing of time man continued to learn about the effects of fire on stone. He would have been well aware from quite early on that when heated some stones changed colour.
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 of heat on rocks and minerals. He documented that 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 when exposed to moisture saying that depending on texture some react differently when wet and dry. He noted that volcanic glass (obsidian) became porous when burnt and that colour and density also changed. The stone known today as amethyst plus many others was noted to change colour when heated.
Theophrastus went on to classify stones according to how they reacted when heated.
Pliny the Elder Roman author naturalist and philosopher also wrote about the effects of heaing stones saying that one gemstone could be changed into another and the colour of crystal quartz could be changed into that of an emerald. These statements make it clear that he was aware that with the application of heat certain rocks and minerals changed or lost colour.
A variety of heat treatments are used to both enhance and change the colour of gemstones and whilst the results in some may be relatively subtle, in others it can be quite dramatic. Although blue topaz occurs naturally it's extremely rare so the vast majority is produced by heating colourless topaz. Depending on the type of treatment that's used it produces the London, Swiss or Sky Blue varieties.
The gemstone amethyst is often heated to change the colour to yellow which mimics citrine. Natural citrine is quite rare which makes it expensive. When the greener shades of aquamarine are heated the colour changes to a light shade of blue. Blue aquamarine gemstones are more popular for use in jewellery. The mineral tourmaline which can be quite dark becomes lighter when heated.
Most of world's finest rubies and sapphires are heated to improve colour, clarity or both. Gemstones which have not been heated are considered to be extremely unusual. Most tanzanite gemstones are heated to change or enhance the depth of colour and also to remove undesirable tints of yellow or brown.
The primary reason for heating gemstones is to maximise beauty which makes them more desirable hence the value increases.