One of my favorite gemstones is opal. People recognize opals by their characteristic play of rainbow like colors. You need to be careful with opals because opal is a relatively soft (5.5-6.5) stone and they are sensitive to pressure, hard knocks, and heat. Opals are 3-30% water and they can dehydrate if stored under improper conditions.
There are several types of opal –
White opal is white or a very light color with characteristic color play
Black opal – precious opal with char color play – dark gray, dark blue, dark green, or dark gray black. Black opal is less common than white opal
Boulder opal – precious opal with a dark base and color play. It occurs as pebble rock where opal fills in hollows in another stone
Jelly opal is bluish-gray precious opal with minimal color play
Crystal opal – transparent with strong color play on a colorless, vitreous (oily looking) surface
Harlequin opal – transparent to translucent precious opal with mosaic like colors patterns
Common opal is opaque with no color play – also called Potch. Opal is found in Australia, Brazil, central America, Mexico, Russia Nevada and Idaho
Reference: Gemstones of the World – Walter Schumann; published by Sterling Publishing NY
Some other notes on opals: Fire opal has an orange color in matrix and is very sensitive to any stress. Fire opals are found in Australia, Brazil, Guatemala, Mexico, US (Nevada and Idaho), and Russia.
In the USA, Nevada is known for its colorful black opal. The Virgin Valley opal beds in northwest Humboldt County are perhaps the most famous gemstone locality in Nevada. Fossilized wood opal that dates from Miocene epoch also can be found in the Virgin Valley Opal area in Humbolt County, Nevada. Quality precious opal with a multihued rainbow of color is found replacing wood or other plant material in this remote location.
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