This mineral shows a variable interplay of internal colours, and though it is a mineraloid, it has an internal structure which makes it diffract light. Depending on the conditions in which it is formed, it can take on many colours. Precious opal ranges from clear through white, grey, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the black opals are the rarest, whereas white and greens are the most common. Opals vary in optical density from opaque to semitransparent.
Deposits of precious opal can be found for example in Australia (South Australia), Africa (North Shewa Province in Ethiopia) and America (Northern Nevada in the United States).
Source: en.wikipedia.org/wiki/Opal - 23.10.2017