A good and colorful opal usually contains 4 to 5% water, but some may contain up to 30%. If there is over 20% of this trapped water, it will cause cracking and/or crazing, just like when water leaves mud. These precious stones are formed from lumps of silica and are actually silicon dioxide and water. The water in the stone cannot escape unless the opal is heated to extreme temperatures.

Most opal is more than 60 million years old and is typically found where hot springs once flowed. The silica in the springs lined the walls of cracks, vents, and cavities in the bedrock. When the hot springs dried up, the silica hardened into opal. An opal has an extraordinary ability to refract light and reflect specific wavelengths of light. This capability is so unique that the term "opalescence" was coined to describe it. Each tiny sphere of silica within an opal refracts a single pure spectral color depending on the size and spacing of the sphere. Looking at an opal can be like looking at water droplets in a rainbow.

Up to the first half of the 19th century, opals were relatively rare. But then their career boomed suddenly and made them one of the most popular gemstones. In the era of Art Deco opals experienced their flourishing, with contemporary gemstone artists preferring them to all other stones because of their subdued charm, which in turn was excellently suited to be combined with enamel, another very popular material of those days.

Opals are considered to be very magical. They are reputed to have healing powers and are used for various rituals. Wearing a black opal near the heart is said to ward off evil and protect travelers. Arabs believed that opals have a fiery color because they fell from heaven in flashes of lightning. Opals were set in the crown of the Holy Roman Emperor and in the crown jewels of France, and it is said that Cleopatra wore an opal to attract the gaze of Mark Antony.

The opal has been mined for centuries, at least since Roman times when they extracted the opal from areas now within the Czech Republic. Archeologists have found 6,000-year-old opal ornaments in African caves. The Aztecs made use of local Mexican sources as did the Spaniards when they exported the material back to Europe. Although opals are found in Brazil, Indonesia, Canada, Ethiopia, Austria, Honduras, Mexico, Nevada, Idaho, Oregon, and Mojave, 90% of the world's gem-quality opals come from southern Australia and ALL black opals come from Australia.

The history of Australian Opal began millions of years ago, when parts of Australia were covered by a vast inland sea, and stone sediment was deposited along its shoreline. When the water masses flooded back, they flushed water containing silica into the resulting cavities and niches in the sedimentary rocks, and also the remains of plants and animals were deposited there.

Australia's lost sea is the only place in the world known to harbor animal fossils in the opal. It is extremely rare for conditions to be right for formation of fossils; and even more rare for opalised fossils to form. Usually, only the hard parts of living things fossilize — for example seed pods, wood, teeth, bones and shells. This often happens after the plant or animal (or a part of it) is buried in sand or other sediments that slowly turn to stone.

Some scientists think opalised fossils (and other opal) took thousands of years to form, at high temperatures and under great pressure; others think opal formed quickly, at about 20 degrees Celsius. Opalised fossils formed when animal or plant parts entombed in stone were replaced by silica, in the form of opal. Opalised bones, teeth, shells and pine cones are rare and dazzling reminders of a time, 110 million years ago, when dinosaurs and other strange reptiles ruled the land, waters and skies around a great inland sea covering nearly one third of Australia.

Pieces of dinosaur eggshell and even an imprint of dinosaur skin have been found preserved in opal. And - very occasionally — an ancient trail of dinosaur footprints is found marching, ghost-like, across the sandstone roof of an opal mine. Dinosaur teeth, bones and claws are among the most exciting of opalised fossils. Plesiosaurs, pliosaurs and ichthyosaurs were swimming reptiles that lived in Australia's inland sea. They were streamlined fish-eaters — the ancient reptile world's equivalent to dolphins. Their opalised bones and teeth are found at opal fields such as Coober Pedy, White Cliffs and Lightning Ridge.

Lightning Ridge in northern New South Wales produces opalised fossils of a variety of dinosaurs including small, fast-moving carnivorous dinosaurs; large, plant-eating sauropods; and agile, long-legged hypsilophodontids that cropped plants with sharp beaks. Dinosaur fossils are rare in Australia and the opal fields are an invaluable source of information about what the world was like when dinosaurs ruled the land.

From LostSeaOpals.com
(Contributed by Diane in New South Wales, Australia)