Opal is an amorphous form of silica related to quartz. It is a mineraloid and not a mineral since water forms 3% to 21% by weight. Opal is the national gemstone of Australia, which produces 97% of the world’s supply. South Australia alone meets 80% of the world’s demands.

Opal diffracts light due to its internal structure. It can take on many colors depending on the conditions in which it was formed. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown to black.

Of these hues, the reds against black are the most rare, whereas white and greens are the most common. Its optical density varies from opaque to semi-transparent. While used as a gemstone, thin layers of opal are placed on basalt – a darker underlying stone, enhancing the natural colors of the opal.

The other kinds of common opals are:

The ‘milky opal’ existing as milky bluish to greenish variety.

‘Resin opal’ a honey-yellow colored opal with a resinous luster.

‘Wood opal’ is formed when the organic material in the wood is replaced by opal.

‘Menilite’ is a brown or grey colored opal.

‘Hyalite’ or ‘Muller’s Glass’ is a colorless clear opal.

‘Fire opals’ are the transparent to translucent opals with warm body colors of yellow, orange, orange-yellow or red. They do not usually show any play of colors but may exhibit bright green flashes. The most famous source of fire opals is Mexico. These are commonly called Mexican fire opals.

‘Jelly opals’ are Fire opals that do not show play of color.

‘Cantera Opal’ is Mexican opal cut in its ryholitic host material.

‘Mexican Water Opal’ is colorless and exhibits either a bluish or a golden internal sheen.

‘Girasol opal’ is used to refer to fire opals and to a type of milky quartz from Madagascar. This transparent to semi-transparent milky quartz displays an asterism or the star effect, when cut properly.

‘Opal-AN’, a Girasol Opal which is a type of ‘Halite Opal’, exhibits a bluish glow or sheen that follows the light source around. It is not a play of color as seen in precious opal but rather an effect from microscopic inclusions.

‘Water Opal’ is the name given to this opal when it comes from Mexico. The two most notable locations of this type of opal are Oregon and Mexico.

‘Peruvian opal’ (blue opal) is a semi-opaque to opaque blue-green stone found in Peru which does not display pleochroism. Blue Opal also comes from Oregon and Nevada.

‘Pineapple opal’ is a rare form created by the deposition of opal in various fossils. It is found only at White Cliffs, New South Wales.