Opal – pronounced (ō‘pəl) is the modern birthstone for October. A natural hydrated form of silica. Opal is a relatively common mineral in its common form, which is known as common opal and lacks the play of color for which gem, or precious, opal is known. All opal is of relatively simple chemical composition, SiO2 · nH2O. The hardness of opal on the Mohs hardness scale ranges from 5 to 6.
The color of common opal ranges from transparent, glassy, and colorless to white and bluish white. The luster of the stone can go from vitreous to pearly. Precious opal has a play of color that is the result of white light being diffracted by the relatively regular internal array of silica spheres. Semiprecious Opal is beautiful but it does not have the fire and brilliance of a precious opal. Because opal is a hydrous mineral, certain opals from specific geologic occurrences may crack because of water loss. Therefore, considerable care is required in the polishing and handling of opal.
Several trade terms are used to describe the appearance of precious opal based on transparency, body color, and the type of play of color. Some of these terms are black opal, which is translucent to almost opaque, with dark gray to black body color, with play of color; fire opal, which is transparent to semitransparent, with yellow, orange, red, or brown body color and with or without play of color; harlequin or mosaic opal, in which the play of color occurs in distinct, broad, angular patches; and matrix opal, which consists of thin seams of high-quality gem opal in a matrix.
The name opal probably is derived from the ancient names for the stone. Sanskrit; upala which translates “precious stone”, the Greek; opallios which translates “color change”, and the Latin; opalus which translates “stone from several elements”. It has been mined for centuries, at least since Roman times when they extracted the opal from areas now within the Czech Republic. The Aztecs made use of local Mexican sources as did the Spaniards when they exported the material back to Europe.
Australia produces around 97% of the world’s opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% and all the opal fields produce white opal; Crystal opal or pure hydrated silica makes up 30%; 8% is black and only 2% is boulder opal.
The town of Coober Pedy in South Australia is a major source of opal. Common, water, jelly, and fire opal are found mostly in Mexico and Mesoamerica. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color).
Boulder opal is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.
A source of white base opal in the United States is Spencer, Idaho. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.
The state gem stone for Nevada is precious black opal, which is named for the true black opal found in Virgin Valley, Humboldt County, Nevada.
OTHER TYPES OF OPALFire opal is a translucent to semi-opaque stone that is generally yellow to bright orange and sometimes nearly red and displays pleochroism at certain angles.
Peruvian opal is a semi-opaque to opaque stone found in Peru which is often cut to include the matrix in the more opaque stones. It comes in various shades of blue and pink. It does not display pleochroism.
African Opal is a semi-opaqie to opaque stone found in Africa. It is found in various shades of white, yellow and brown.
The Traditional birthstone for the month of October is Tourmaline.
The two Zodiac signs that fall into October are Libra and Scorpio. For Libra the birthstone is Peridot or Chrysolite and for Scorpio it is Aquamarine.
For ages people have believed in the healing power of Opal. It is reported to be able to solve depressions and to help its wearer find the true and real love.
Opals are supposed to further enhance the positive characteristics for people born under the zodiac sign of Cancer. Black Opal is recommended to those born under Scorpio, and Boulder Opal is the lucky stone for Aries.
With the deep scoop dangling earrings will go well but keep away from the very dramatic ones 
