The effect of heat treatment on zircon inclusion in Madagascar Sapphire

Zircon is a common inclusion in Madagascar sapphire, which is important because they span the color spectrum. During heat treatment, included mineral crystals are subjected to the same condition as the corundum host, but since they have different thermodynamic and chemical properties, they may expand, recrystallize, melt observation of destroyed or modified mineral inclusion using an optical microscope has long been used to identify heat-treated sapphire

Unheated samples, the unheated zircon inclusions were transparent with a slightly elongate crystal habit and well-developed {110} form, although many were slightly rounded, displayed cluster. The crystal faces of the zircons were flat or slightly rounded and reflective. Some of larger zircons crystal and clusters were accompanied by redial fracture in the surrounding corundum. The surface showed uniform zircon crystal with sharp planar or slightly rounded boundaries and no evidence of decomposition; a few displayed growth zoning.
Heat samples observation the zircon inclusions displayed distinct changes in crystal morphology after heating to sufficient temperature. Below 1550celsius the euhedral morphology general remained unchanged. At temperatures as low as 1400 celsius, portion of the interface between some zircon and the host sapphire became slightly turbid or frosted, though most of the body of the zircon remained clear and transparent . Temperature increased to 1550 celsius most of the zircon inclusions were noticeable affect. After heating at 1600 celsius, the zircon inclusion display even more distinct morphological variations. While the overall outline remained fundamentally unchanged. Temperature 1680 celsius the shapes of the inclusions became irregular, losing their euhedral outline. Temperature 1850 celsius melt-filled fracture showed basically the same orientation within the sapphire host and were nearly parallel to each other.
Application for gem identification, visual evidence of modification or distruction of solid inclusion in sapphire and ruby are powerful clues for determining if a stone has been heated. The results form this study can be used to determine not only the possibility of heat treatment at sapphire with zircon inclusions, but also estimate of temperature applied.