Geuda (, pronounced gay-yoo-dah) is a form of the mineral

corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium, and chromium. It is a rock (geology), rock-forming mineral. It is a naturally transparency and translucency, transparent material, but ...

, or

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...

, found primarily in

Sri Lanka Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian subcontinent, ...

. Around 70%-80% of gems mined in Sri Lanka belong to geuda varieties. Because of its semitransparent and milky appearance due to

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at vis ...

inclusions, these stones have little value as gemstones in their natural state. Geuda was frequently stored in large drums or used to gravel home gardens prior to the 1970s discovery that heat treatment can drastically alter the stone's color.Stephen M Voynick, Yogo: The Great American Sapphire. Mountain Press Publishing Company, Missoula. 1985 Some geuda varieties turn to a blue color after heat treatments. Others turn to red after oxidizing. Kowangu pushparaga turns to yellow sapphire after oxidizing. After heating geuda to roughly 1800 °C, the

aluminium oxide Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several Aluminium oxide (compounds), aluminium oxides, and specifically identified as alum ...

lattice-work of the gem is disrupted and cooling greatly improves both color and clarity. Though many stones are destroyed by the heating and cooling process, those that survive are significantly altered and rival naturally blue sapphires in both appearance and price.

References

Corundum gemstones Sinhala words and phrases {{mineral-stub