A little about Opals
There are many different types of natural opals but there are 4 main types of Precious opal; Black, White, Crystal and Boulder opal. The other kinds include- fire opal, hyalite, water opal, hydrophane, jelly, common (potch) and honey opal.
Opal is found all over the world in different forms. Australia is the most famous producer of opals with the majority if the traditional market being here. In recent years large deposits of opal have been found in Ethiopia and are making up a growing portion of the market. other places include Brazil, Honduras, Mexico Northern USA, Canada and europe.
Precious Opal - is opal which exhibits the phenomenon known as play of colour, which is produced by the diffraction of white light through a micro-structure of orderly arrayed spheres of silica.The internal structure of precious opal makes it diffract light; depending on the conditions in which it formed, it can take on many colors.
Black Opal is the most valued of Opals and comes mainly from Lightning Ridge. High quality stones are very rare. The term 'black opal' does not mean that the stone is completely black (a common mistake), it simply means the stone has a dark body tone in comparison to a white opal. Easily distinguished by the blackness of the background "body tone" or body colour.
White opals with a base tone ranging from colourless to medium grey are called light opal. Some people refer to these as "white" although this expression should only be used where the body colour is very milky. White opal is more common and because of its body tone, generally does not show the colour as well as black opal. Nevertheless, white opals can still be absolutely magnificent in colour if a good quality stone is found.
Crystal Opal can be black, white, or from boulder opal. This means that there is no host rock or "potch" on the back. Black crystal opals are among the most valuable available. Held up to the light these opals may appear to be an amber colour.
Boulder is a variety of precious opal that has the host rock forming naturally as part of the gem. Often just a thin vein of precious opal is present. It mainly occurs in specific locations over a wide area of Western Queensland. Boulder opal occurs as in-fillings of cracks or voids usually in ironstone boulders. Boulder opal can be black or light depending on the appearance of the stone when viewed from the surface. Boulder Opal has a tendency to cleave; when cleaved the "split" leaves two faces of opal, with a naturally polished face. Boulder opal has a sub group known as Matrix opal.
The term matrix opal is commonly used where the opal is intimately diffused as infillings of pores or holes between grains of the host rock in which it was formed. Boulder matrix opal is found in Queensland and can be distinguished by the ironstone host rock. Andamooka matrix opal is a porous material from Andamooka,South Australia, Which may be enhanced by soaking the specimen in a sugar solution and then boiling in acid to deposit carbon in the available pore spaces, resulting in a dark background.
