Heat Treatment and Thermal EnhancementFor Centuries - as common trade practice - gemstones have been treated by man in different ways. This is also the case with rubies and sapphires.
For natural rubies and sapphires the most common treatment is thermal enhancement or heat treatment.
Heat treatment is done to improve color or clarity or both of the ruby or sapphire, by altering the chemical structure of the stone. Originally heat treatment was done by using simple tools and charcoal burning. Todays methods are sophisticated often heating the gemstone from 1500- 1800 degrees Celsius in special gem ovens.
Conditions for heat treatment will vary depending on the individual gemstone. The ruby crystal may have a blue hue; it may be too brownish or have an uneven distribution of color.
Heat treatment will tend to improve the overall color appearance of any such stone. The time it takes and the temperature required depend on the stones. 7-14 hours at 1700 degree Celsius is quite common.
Heat treatment may also improve the clarity of a ruby by dissolving natural inclusions e.g. the fine rutile needles or silk structures.
For the same reason as the treatments tend to improve the general appearance of the stone, detection and identification of heat treatment is possible for the trained eye. The heat treatment leaves traces, which modify the natural inclusions often by rupture of any fluid inclusions or partial dissolution of mineral inclusions. Or in case of "silk" in the stone, by leaving the original pattern diffused and broken.
Although appearance improves the high temperatures may cause the ruby to become more fragile and brittle. It also looses its individuality and becomes dull as if produced by a machine. Finally and most importantly, it looses much of its commercial value.
Natural or un-treated rubies are becoming extremely rare in todays market place and are therefore more desirable and valuable.
Untreated rubies are often accompanied by a gem report or certificate confirming that the stone has no evidence of heat enhancement.