Black, Fire

Opal:
Color: An orangy crystalline glow if put up to light, but when held in hand runs the gamut from opaque white to crystalline orange. All opals have a background color and texture upon which brilliant color plays in ever-shifting patterns are shown, in every color of the rainbow.
Mohs hardness: 5.5 - 6.5
Refractive Index: 1.44 - 1.47
Countries of Origin: The oldest opal mine was at Czerweniza, in the former Czechoslovakia. There is evidence that this mine was worked in the 14th century, but it may in fact have been in operation centuries earlier-- as the source of stones for Rome. The black opal was first founded in Australia in 1887.
Today Australia produces 85% of the worlds opal. and the distinctive patterns which the color patches make on the picture plane of the stone. Opal is formed in the cavities and cracks of near-surface volcanic rocks. It is also created by percolating water in or near sedimentary volcanic ash that dissolves silica of shells, bones and woods, fossilizing them into opal.
In History, Literature and Lore: Opal gees from the Sanskrit upala and the Latin opalus, meaning precious stone. The opals principal characteristic is its play of beautiful colors. The Romans believed that opal symbolized love and hope; the Arabs, that it fell from the heavens in flashes of lightning. Queen Victoria helped popularize the stone by giving it to her children.
It was a favorite stone of Rene Lalique, the noted artisan of the Art Nouveau movement, who designed opal jewelry for the great actress Sarah Bernhardt. The French believed opal rendered the wearer invisible, making it known, too, as the talisman of thieves and spies. And Australian legend had that the starts are governed by a huge opal that also controls gold in the mines, and human love. The aborigines, however, felt that opal was a devil-- half-serpent, half-human, luring men with evil magic.
What to Look For:
Opals are judged by the range of bright colors which play on the surface in ever-shifting patches on the picture-plane of the stone. The hot colors-- such as red, orange or violet-- in the color flashes are the most prized. The stones patterns can even have names, such as a harlequin pattern, or rolling flash, or pinfire. Plant and silica-based polymers are occasionally used to improve the appearance of Brazilian, Mexican and Idaho opals.
The beauty of opal helps to outweigh its less than optimal physical properties. These stones are very soft and heat sensitive, and great care should be taken in cleaning. Sudden changes in temperature should also be avoided. As a ring-stone , it should not be worn every day as it can chip. This gem contains water, which may evaporate, leaving it slightly smaller, stressed and cracked.Care should be taken not to purchase opals which have already cracked (or checked) from dryness.
Black Opal:
A black background inherent in the rough that offsets the color play of the opalescent material. The background can range from grey to charcoal to matte black; the opalescent material runs the whole gamut of the rainbow.
Country of Origin: Australia.
What to Look For:
Black opal is considered the king of opal materials. Its natural dark background greatly enhances the color play of the opalescent material. The blacker the background, the better, with red and orange greatly prized. Buyers should beware that there are some white Australian opals are treated to make them black. After soaking in a sugar solution, they are immersed in sulfuric acid, which carbonizes the sugar and blackens the stone.
Mexican opals, too, have been turned black by smoke treatment in a mixture of charcoal and cow manure. Opal doublets are also manufactured as a lookalike for black opal by sandwiching black onyx and opal.
Fire Opals:
Transparent stone from white(play of color may or may not appear) to translucent yellow, bright orange, red or brick. Origin: Important deposits are in Mexico (Hidalgo and Queretaro provinces).
What to Look For:
These are the most vibrant of all the orange-colored stones in the gem world, but also the most fragile. They are not suitable for rings but are perfect for pendants. The opalescent types are very beautiful with bright orange background and transparent opalescent patterns throughout the gemstone, and they are the most valuable. Fire opals are the only of the opal materials used for faceting.
Jewelry
Other guides relating to jewelry and gemstone buyingwhich you may find helpful are as follows:


Gold Prices and Gold Jewelry