Historically, many gemstones have been enhanced to improve their appearance. At the present, a variety of traditional and advanced enhancements are applied to many natural gemstones such as beryl, corundum, tourmaline, topaz, zircon, zoisite, etc. Heat treatment (also known as thermal enhancement) is applied to gemstones such as rubies & sapphires to improve colour and /or clarity (maybe some phenomena such as asterism). Thermal enhancement of rubies and sapphires is considered stable and permanent, and it is generally accepted by the international gem and jewellery trade.
             The process of thermal enhancement in rubies and sapphires is performed to improve the colour and /or clarity. This process may incorporate the use of chemicals such as sodium tetraborate powder, in order to help facilitate.The closure of fractures by healing or infilling and protect the stones from tension crack which may happen in high temperature condition.
            Upon exposure to elevated temperatures, the chemicals will melt and coat the surface of the gemstone and enter fractures. When cooling condition start, chemical melted during the heating process will solidify as an amorphous solid. This material maybe encountered at the surface of polished gemstone and remain trapped within areas of the fractures of partially healed by the other way, the amorphous solids also may result from the  thermal alteration and interaction of naturally occuring inclusions within the host gemstone.

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Ruby - Sapphire - Gemstones
Ruby - Sapphire - Gemstones
Ruby - Sapphire - Gemstones

Thermal Enhancement In Rubies & Sapphires