Opal Mineral Facts:

Chemical Formula: SiO₂: n H₂O Hydrated silicon dioxide.

Colors: Usually some shade of white, yellow, red, green or blue. Streak is white.

Hardness: 5.5 to 6.5

Density: 2.1

Cleavage: None - Conchoidal fracture.

Crystallography: Amorphous

Luster:. Vitreous, translucent to transparent. 

Optics: (Refractive Index): n = 1.44

Composition, Structure and Associated Minerals:
Opal is found lining and filling cavities in igneous and sedimentary rocks, where it has evidently been deposited through the agency of flowing waters. In its ordinary variety it is of widespread occurrence.

The true position of opal in the classification of minerals is somewhat doubtful and some would say that technically it is not a mineral as it has no fixed chemical make up or crystalline form. From the analyses made it appears to be a combination of amorphous silica and water, or, perhaps, a mixture of silica in some form and a hydroxide of silicon. The percentage of water present is variable. In some specimens it is as low as 3 per cent, while in others it is as high as 13 per cent. The mineral is not known in crystals. It is a colloid, in which the water is, in part at least, mechanically held in a gel of SiO2- It occurs only in massive form, in stalactitic or globular masses and in an earthy condition. It is a common alteration product of silicates, and seems to have been deposited from both cold and hot waters.

Identification and Diagnostics
Infusible and insoluble, opal reacts like quartz but gives alittle water upon intense ignition in closed tube. Being amorphous, opal is isotropic under the microscope. This means it remains black in all positions when rotated between crossed nicols.

Occurrence, Localities and Origins:
The principal varieties of opal are:  Precious opal, a transparent variety exhibiting a delicate play of colors, Fire opal, a precious opal in which the background colors are quite brilliant shades of red and yellow, Common opal, a translucent variety without any distinct play of colors, Hyalite, a transparent, colorless variety, usually in globular or botryoidal masses,
Wood Opal is wood in which the cavities have been filled and the tissues replaced by opal. Siliceous sinter, white, translucent to opaque pulverulent accumulations and hard crusts. Sometimes deposited from the waters of hot springs. This material is found around the hot springs of Taupo, in New Zealand, a the geysers of Iceland and parts of the United States.

Precious opal is found near Kashan, in Hungary; at Zimapan and Quaretaro, in Mexico; in Honduras. The most important opal mines are located in Australia, including Queensland and New South Wales. Opal exhibiting intense fire on a dark background has been mined in Humboldt County, Nevada. Siliceous sinter is deposited at the Steamboat Springs in Nevada and at the mouths of the geysers in the Yellowstone National Park. The precious and fire opals are popular and handsome gems. Opalized wood, i.e., wood that has been changed into opal in such a manner as to retain its woody structure, is often cut and polished for use as an ornamental stone.

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