The art of "cooking" gemstones is almost as ancient as the gemtrade itself. the earliest known reference is found in Pliny´s (23-79 AD) monumental work Historia Naturalis.
A thousand years later, in the 11th century, Arab polymath Abu Raihan al-Biruni published Kitāb al- Ǧamāhir fī maʿrifat al-ǧawāhir min taṣnīf al-ustāḏ, at the time the most influential book on minerals and gem stones, in which he describes the heating of rubies to lighten their colours.
Finally, in the 20th century, when ovens capable of reaching 1500°C and more became available, heat treatments started to be performed widely and routinely.
However, it does not always take extreme temperatures to change or improve gem stone colours.
Many stones like quartz, tourmaline, aquamarine, zircon or tanzanite react at relatively low temperatures of 400 to 600°C. In some cases the changes are quite dramatic. Thus amethyst becomes citrine, brown zircon turns petrol blue and the uncomely grey-brown tone of unheated tanzanite turns into an attractive and highly sought-after purplish blue.
Besides improvement of colour "cooking" has another effect which is often ovelooked: it makes a lot of gem stones much more affordable!
Now, many customers prefer unheated (or, in general, untreated) gems but in most cases those are very rare and substantially more expensive.
So, we have nothing against the "cooking" of gem stones, quite the contrary, as heating gems has some significant advantages over other colour-enhancing treatments.
Colours produced by heating are stable. Other than some colours produced by irradiation e.g. in some yellow sapphires and some topaz, they do not fade upon prolonged exposure to UV light (sunlight).
The change of colour effects the whole gem and not just a few tenths of a millimeter as is the case with diffusion treatments. Thus a heat-treated stone can be re-polished or re-cut slightly to fit into a setting. As long as you don´t grind away too much, colour will not be affected.
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