Sapphire is a precious gemstone, a variety of the mineral corundum, an aluminium oxide (α-Al2O3). It is typically blue, but natural "fancy" sapphires also occur in yellow, purple, orange, and green colors; "parti sapphires" show two or more colors. The only color which sapphire cannot be is red – as red colored corundum is called ruby, another corundum variety. Pink colored corundum may be either classified as ruby or sapphire depending on locale. This variety in color is due to trace amounts of elements such as iron, titanium, chromium, copper, or magnesium.