The internal structure of precious opal makes it diffract light; depending on the conditions in which it 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 reds against black are the most rare, whereas white and greens are the most common. It varies in optical density from opaque to semi-transparent. Common opal, called "potch" by miners, does not show the display of colour exhibited in precious opal. Besides the gemstone varieties that show a play of colour, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality). other variaties are:- Fire opal is a transparent to translucent opal, with warm body colours of yellow, orange, orange-yellow or red. It does not usually show any play of colour, although occasionally a stone will exhibit bright green flashes. These opals are commonly called Mexican fire opals. Girasol opal is a term sometimes mistakenly and improperly used to refer to fire opals as well as a type of transparent to semi-transparent type milky quartz from Madagascar which displays an asterism, or star effect, when cut properly. Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. Australia produces around 97% of the world's opal. 90% is called 'light opal' or white and crystal opal. White makes up 60% of the opal productions but cannot be found in all of the opal fields. Crystal opal or pure hydrated silica makes up 30% of the opal produced, 8% is black and only 2% is boulder opal. In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose colour was represented in the colour spectrum of the opal. It was also said to confer the power of invisibility if wrapped in a fresh bay leaf and held in the hand.