Corundum:Corundum is a crystalline form of aluminium oxide (Al2O3) and is the second hardest natural mineral after diamond. It includes all natural gemstones that are transparent and have any colour other than red, which are classified as ruby. Sapphires can be any colour except red, with blue being the most popular and valuable colour. Other colours include yellow, green, orange, pink, purple and white. Corundum is found in metamorphic rocks such as gneiss and schist. It is also found in alluvial deposits where it has been eroded from metamorphic rocks and transported by rivers.