Opal is a natural silica colloid containing 5-10% water. Unlike most gemstones, opals are amorphous. There are some unstable amorphous structures in opals that change color or even crack due to the lack of moisture, but there are also opals with very stable structures that are unchanged in normal environments. Opals are generally egg white and can form various colors if other atoms are mixed in, such as those containing iron, calcium, magnesium and copper, which usually appear blue, green, yellow, red, etc. Australian variegated opals, which combine the colors of various gemstones, are sought after by Westerners as one of the top six gemstones because of their strong variegated color effect. Opals generally have a glassy or waxy luster.
