Corundum is a crystalline form of aluminium oxide (Al
2O
3) typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen.