Opal is one of only six types of precious gemstones found on the earth, along with diamonds, rubies, sapphires, emeralds and pearls. Over 95% of the world's precious opal comes from Australia, and it is Australia's national gemstone.
Although there are several different types of opal present in nature, there are only two main varieties – precious opal and common opal. When common opal is found in association with precious opal, it is known as potch. Common opal and potch are basically one coloured – white, grey or black – and make up about 95% of all opal mined from the opal fields. The remaining 5% is precious opal, and only a small percentage of this has any real value.
Precious opal is mainly red, blue and green, with red being the most desirable. Opal is formed in the cavities of volcanic rocks when water drips slowly down through silica. It is formed in the sedimentary rocks of Australia's Great Artesian Basin and is basically a mix of silica dioxide and water under pressure. The water seeps through the sandstone, picking up the smallest particles of silica on its way The interference and diffraction of light passing through these tiny silica spheres in the structure of the opal give rise to the 'play of colour' as the light is split into the colours of the spectrum, appearing to move and change with the angle of view. This is what makes this comparatively rare gem so valuable
Black Opal is known for its rich, dark appearance. The opal itself is not black, but the dark to very dark tone of the base rock emphasizes the brilliance of opal's colours. It is the rarest and so generally the most valuable of all the opals. Australian Black Opal is uniquely beautiful and is becoming more and more scarce, with production at an all-time low. It is highly sort after internationally. Black Opal is primarily mined in and around the Lightning Ridge region of north-western New South Wales.
Boulder Opal, as its name suggests, forms naturally in the narrow cavities and crevices of dark, ironstone boulders. The opal generally occurs as thin, often wavy veins naturally attaching to the ironstone, which, similar to the black opal, can make the opal appear dark. When cut and polished the piece retains the ironstone on the underside, some of which might be visible in the surface of the polished gem. If there are no ironstone intrusions it is called 'clean faced', usually adding to its value and fetching a higher price. Boulder Opals can vary dramatically in size and are mined in vast isolated regions of central and south-western Queensland.
LIGHT OPAL (WHITE OPAL AND CRYSTAL OPAL)
Light Opal refers to both White Opal and Crystal Opal. They are mined in South Australia at many opal bearing areas including Coober Pedy, Andamooka, Mintabe and Lambina
White Opal has a natural milky-white or translucent appearance, often making the colours harder to see. By quantity, more White Opal is mined than any other type of precious opal. Its abundance and its general lack of sheen make it usually less valuable than Black Opal. Its value lies in the fact that it is formed in horizontal seams, making it easy to mine and to cut to standardized sizes without wastage. As with all types of precious opal, the quality of White Opal can vary greatly, with some displaying almost as many tones and colours as the black.
Crystal Opal has a transparent or semi-transparent glass-like appearance. It is also unusual in that it can appear in either a light or dark tone. Imagine the effect of a semi-transparent Black or White Opal! A high quality Crystal Opal will be the most expensive of all the precious opal types.