Heat Treatment

Geuda is a variety of gemstones in the corundum species of minerals. Corundum which possesses a deep red variety known as ruby and many other colors such as blue, pink, yellow, orange, green, reddish orange (known as Pathmaraga) is referred to as sapphire with a prefix for color. It is only second in hardness to that of diamond but it certainly surpasses diamond in beauty, abundance and appeal due to its spectral colors. Other lesser-known varieties of corundum are star sapphire, star ruby, 'Geuda' and 'Ottu'. Geuda and Ottu varieties constitute about 30 - 40% and 5 - 10% respectively of the total corundum stones found in Sri Lanka.

Geuda are not attractive enough to be cut and polished and set in jewelry, as they translucent-yellowish or in the color of diesel in transmitted light (looking through the stone) and milky or silky (reflective) in reflected light (appearance when stone is placed on the palm) with the basic body colour being light blue, red or yellow. The Ottu on the other hand is colorless corundum with a patch, a dot or a streak of blue.

While Ottu always produce transparent blue colored sapphires Geuda may turn in to transparent blue, red, yellow or orange depending on their body color on heating to around 1,700 degrees Centigrade. What in effect is happening in Geuda on heating is that sub-microscopic scale rutile (TlO2) needles that causes translucency, dissolve into the basic corundum (Al2O3) composition creating an appearance similar to that of naturally occurring sapphire. We can say that man takes over where the nature has stopped and is doing it in a day or two as against millions of years; it would have taken for the same to happen naturally. - (Professor P. G. R. Dharmaratne)

At Amaran, heat treatment is done by using the “ Lakmini Furnace” and heat resistance electrical furnace.

Lakmini” furnaces are used to convert Geuda (Semi Precious stones) into high value precious stones such as Blue sapphires, White Sapphires, Rubies and Yellow Sapphires while treating under either reducing or oxidizing conditions. Temperature up to 1950 °C can be achieved and unique design features facilitate effective temperature control and this helps to give desired time and temperature combinations for different types of gems.