Called the "Rainbow Stone" by the Aborigines, the opal is one of nature's most striking gifts. This precious gem has the ability to expose an infinite number of colors, constantly moving with the light. The rare black opal is recognised as one of the world's outstanding gemstones with Australia having 97% of the world's supply with the mines at Lightning Ridge producing the largest number. The elusive black opal is the most sought after as the dark background accentuates the brilliance of color. Opal was formed thousands of years ago when liquid silica filtered down into the faults and fissures of sedimentary rock. When the water content evaporated, tiny spheres of silica remained and overtime solidified. The other two categories of opal are crystal opal and light opal. The value of an opal depends on the amount and brilliance of the color, preferably being evenly distributed across the face of the polished opal. The greater the spectrum of colors from red to violet the better, with crimsons, reds and orange being rarer than greens and blues. Milkiness, cloudiness or greyness detracts from the value. Basically the greater number of colors, the more prized the gem and if these are arranged in a block pattern, more value is added. Opal mining is an arduous occupation with miners working in tempertures up to 55 degrees celcius and only 10 % of miners find it profitable. When purchasing an unset opal, it is important to consider the way it will be worn. There are many places to buy and receive expert advice about opals in Tropical North Queensland. The greatest selection of shops - several of them duty free - is in Cairns, but there are also several stores in Kuranda, Atherton and Port Douglas. It's also worth checking out opal cutting factories, as they offer very competitive prices, along with the Opal Mine in Palm Cove.