Opal is an hydrated silicum dioxide which reveals rainbow colours which change according to the angle of view. Opals exist in many colours : colorless, white, yellow, orange, red, yellowish brown, greenish, blue, gray, black and violet.

There are several varieties of opal such as : Fire Opal (translucide to transparent, with a yellow, orange or red body color), Milk Opal (translucent with a milk white color), Harlequin Opal (transparent to translucide with color draws simiar to mosaic patterns) and Water Opal (transparent or colorless body that may have fire in it).

The most important deposits are in Australia ; this stone is also found in Brazil, Honduras, Guatemala, Indonesia, Mexico, Japan and in the United States (Idaho and Nevada). Besides, the name “Opal” comes from the Sanskrit (Indo-European language) word “Upala” meaning “precious stone”.

Sources : http://www.gemselect.com/ and https://www.gemsociety.org/