Geuda (ගෙවුඩ, pronounced gay-yoo-dah) is a form of the mineral corundum, or sapphire, found primarily in Sri Lanka. Around 70%-80% gems mined in Sri Lanka belongs to Geuda varieties. Because of its semitransparent and milky appearance due to TiO2 (Rutile), 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. Ottu, Dun, Diesel, Milky, silky are some Geuda varieties which give blue color after heat treatments. Deguna and Red Geuda turn to red after oxidizing. Kowangu pushparaga (pronounced koo-wangu) turns to yellow sapphire after oxidizing. After heating geuda to roughly 1800 degrees Celsius, the aluminum oxide 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.