Opal is from the Greek "Opallos" meaning to "see a change of colour. Each Australian opal is unique and the different varieties offer a spectacular choice of colour, shape and size. When rotated, the opal displays a constantly changing interplay of fiery colours. The finest quality stones can command prices similar to the finest emeralds, rubies and diamonds.

Opal is not only one of the world's most beautiful precious gemstones; it is also one of the worlds rarest. Opals rarity has made it a desirable and at times divisive part of many cultures over thousands of years.

Technically, opal is non-crystalline silica (in a rigid gel form) containing approximately 5% to 9% water. Non-crystalline silica is similar to quartz and sand. On the Mohs Scale of Hardness, a scientific scale of the relative hardness of minerals, opal rates at around 5.5 to 6.5. As a comparison, diamonds rate at around 10, sapphires 9, quartz 7 and pearls 3 - 4.

ommon Opal is found in a limited number of countries. A small amount of precious Opal is found in Brazil and Mexico. However, 96% of the worlds precious opal comes from three isolated areas in the outback of Australia. Australian stones can easily be distinguished by their brilliance and play of colour.

The opal fields of Australia are in the harsh outback where only the hardened miner can make a living.

• Black Opal comes from Lightning Ridge in New South Wales.

• Light or White Opal is mined principally from the South Australian fields of Mintabie, Coober Pedy and Andamooka.

• Boulder Opal is from Central Queensland in the areas surrounding the townships of Quilpie and Winton.

Although experts divide gem opals into many different categories, five of the main types are:

• White or light opal: Translucent to semitranslucent, with play-of-color against a white or light gray background color, called bodycolor.

• Black opal: Translucent to opaque, with play-of-color against a black or other dark background.

• Fire opal: Transparent to translucent, with brown, yellow, orange, or red bodycolor. This material—which often doesn’t show play-of-color—is also known as “Mexican opal.”

• Boulder opal: Translucent to opaque, with play-of-color against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.

• Crystal or water opal: Transparent to semitransparent, with a clear background. This type shows exceptional play-of-color.