gems, corundum, treatments.6, HPHT

For decades now heat treatment has been used successfully as an enhancement process for sapphires, particularly blue sapphires, and is both accepted and widely practiced in the jewellery industry. Considered one of the classic coloured gemstones, fine quality blue sapphire is always in great demand. The techniques used for heat treatment are constantly being developed, not only in Thailand, and seem to involve increasingly sophisticated instruments. For instance, the famous Sri Lankan heat treatment technique known as the “Punsiri method” that has been seen to improve the colour of blue sapphire from various deposits successfully (G. DuToit et al., 2009). In 2013, a Korean company successfully developed a new technique for enhancing blue sapphire and began to trade their product in the gem market. This process involves the intentional application of pressure while heating the stone. This new technique significantly reduces the time needed to change the stone’s colour and/or clarity compared with the traditional heat treatment where stones are simply heated in a furnace. So far, Sri Lankan sapphire, (the so called Geuda sapphires) is the only commercial product that this company claims to be treating successfully. The blue sapphire resulting from this treatment commonly shows a typical strong OH-related absorption in the Mid-IR spectrum. This and some characteristic inclusion features are important keys for identification of sapphire that has undergone heat and pressure treatment. Further study is required to increase our understanding of the change mechanism and also the exact role of pressure in this process.