Opal is a stone that comes in a variety of colors and types, and exhibits a wide range of patterns. Common opal is typically opaque and rarely cut as gemstone; precious opal, however, most of which comes from Australia, displays a property called "play-of-color", where flashes of different colors can be observed when the opal is held under light.
Opal from Australia accounts for more than 90% of the supply of precious opal in the world - in the 1800, Australian opal was discovered, but its market value was relatively low; afterwards, in the beginning of the 20th century, Australian opal's value rapidly increased. It's the country's national gemstone.
Australia produces a few types of precious opal: black opal, light (or white) opal, and boulder opal, among others; those are produced in different parts of the country. Some of the factors that define Australian opal's price are the differences in color and the specific play-of-color of each stone, as well as the pattern it has.
Opal contains water, and the loss of water can result in a partial loss of its play-of-color properties, which is why it needs to be stored under special conditions. Additionally, opal is a gemstone that is brittle and not very hard: with hardness of 5.5 to 6.5 on Mohs scale, it's not suitable for jewelry prone to everyday wear.
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