Opal is the birthstone for October. It shares this distinction with tourmaline.
Opal is a hydrated amorphous form of silica or quartz. I has a water content that can range from 3 to 21 percent by weight.
Chemical formula: SiO2 * nH2O
Hardness: 5.5-6 mohs
Specific Gravity: 2.15
Color: colorless, white, yellow, black, red, orange, green, brown and blue
Ultraviolet fluorescence: black or white body color: inert to moderate light blue, green, or yellow in long and short wave, may also phosphoresce, common to opal: intert to strong green or yellowish green in long and short wave may phosphoresce.
National mineral to Australia.
Precious Opal: Shows a variable interplay of internal colors and although it is a mineraloid, it has an internal structure. It is the most desired for jewelry. Most opal is cut to form cabochons. Opals that are too thin may be combined with other materials, a top and/or bottom to produce a solid opal. These are called doublets and triplets depending on how many levels they have.
Opals are sensitive to heat due to their high water content. Also, never expose the opals to oil as it can displace the water causing irreparable damage to the opal.
Boulder Opal: Forms in ironstone and has veins of opal material running through it. It is usually cut in free forms to maximize the size of the stone.
Mexican Opal: Mexican opal forms in a matrix of rhyolite. It can be cut in either cabochons or as faceted stones.
Honduran Opal: This opal forms in a matrix of basalt. Many times, the opal is formed in needle like structures throughout the stone.
Opals are found in many other parts of the world. Not all opal is riddled with colors and rainbows. Common opal may appear milky or white.
The “Aurora Australis” was found in 1938 at Lightning Ridge and is considered the world’s most valuable black opal. The oval, cut and polished stone has a harlequin pattern with dominant red, green and blue colors against a black background. It weighs 180 cts. and is 3 inches by 1.8 inches.
The Fire Queen: The Fire Queen opal was found 1906 in the Angeldool diggings by Charlie Dunstan. It was nearly 900 carats. Unfortunately, black opals were not popular in the market place so he was only able to sell it to an unknown buyer for 100 pounds. The stone ended up in the collection of J. D. Rockefeller after he paid 75,000 pounds.
The Black Prince: aka: “Harlequin Prince”. Was found in 1915 at Phone Line near Lightning Ridge, Australia by Ted Brown and Tom Urwin. It went on display as a 181 carat dark-gray Opal.
Pride of Australia: aka: Red Emperor. Another opal found by Tom Urwin and Snowy Brown at Phone Line. The 2” x 3” opal has black and blue veins interlaced with brilliant red streaks. 225 carat stone. It along with the Black Prince were stolen in Los Angeles.
Olympic Australis: Found in Coober Pedy in 1956 at the Eight Mile Opal field. Weighing 3450 grams (17,250 carats) it is one of the most valuable and famous opals ever discovered. Valued at over $2.5 million dollars.
Galaxy Opal: This was found in 1976 at the Boi Morto Mine in northeastern Brazil. Original weight was 5,205 carats and it was carved down to 3,749 carats by Scott Cooley.