Opals are found around the world, however Australian opals are among the best known opals. Australia has produced up to 95% of the world's precious opals, and remains the most abundant source of opal to this day. The most common variations of Australian opals are white, blue, black, or boulder opals, which are cut with the natural host rock, ironstone, on the back, and are typically stunning shades of vibrant purple and blue. The most uncommon and rare opal colour is the black opal. The beautiful "opalescence" of opals is caused by tiny silica spheres arranged within the silica-rich matrix of the stone. These spheres diffract light, creating the appearance of colourful flashes within the stone. Most Australian opals are found in Cretaceous age sandstone and mudstone deposits, making opals approximately 66 million to 145 million years old. Interestingly, Australia is the only part of the world where opalized animal and plant fossils have been found, the most famous of which is an opalized Pliosaur (Cretaceous age marine vertebrate) which boasts both the skeleton of the creature, as well as the stomach contents of its last fish meal, all fossilized with opal. Such fossils are valued according to the appearance of the individual specimen and the intensity of the play of colours. Opal has many stories associated with it throughout history, including references from the Romans, Ancient Greeks, Arabs, and even Cleopatra. Opal artefacts several thousands of years old have been discovered in East Africa. There are also many Australian aboriginal dreamtime stories that feature opal. Used in ceremonies and rituals, Indigenous Australians believed opals symbolized a bridge between the earthly and the divine. Others believe that the array and movement of colour within opal may stimulate creativity, empathy, and represent the expansion of the mind. Opal may help stimulate originality and dynamic creativity, encouraging an interest in the arts and one's self-expression. Others believe opal to be a stone useful to create good humour and a good mood; it is useful in creating joy.
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