Category  :  Mineraloid
Hardness  :  5.5–6.5
Gravity  :  2.15 (+.08, -.90)
Reflection  :  1.44 –1.46
Color  :  White, black, red, orange, most of the full spectrum, colorless, iridescent
Chemical formula  :  SiO2·nH2O
Crystal system  :  Amorphous
Major varieties  :  White Precious Opal, Black Precious Opal, Common Or Fire Opal

The word opal comes from the Latin opalus, by Greek opallios, and is from the same root as Sanskrit upálá[s] for "stone", originally a millstone with upárá[s] for slab.

The water content is usually between three and ten percent, but can be as high as twenty percent. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. These color variations are a function of growth size into the red and infrared wavelengths. Opal is Australia's national gemstone.

Precious opal shows a variable interplay of internal colors and even though it is a mineraloid, it does have an internal structure. At the micro scale precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.[5] It is the regularity of the sizes and the packing of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction.

White Precious Opal

It has a whitish (watered-down milk) to light grayish, dull yellow, light blue-gray or pale-blue ground color. The range of colors of the patches, due to diffraction, depends on the size of the minute spheres of which the gem is composed: patches will be violet to blue for structures with very small spheres, gradually turning to green, yellow, orange and red as the size of the spheres increases.

Black Precious Opal

It has a bluish-gray, smoke gray to black color. This dark ground greatly enhances its appearance by emphasizing the patches of color caused by diffraction. The cabochon cut is used and thicker gems are preferred, those of regular outline. When the patches are angular, polygonal, clear-cut and of uniform size it is called as (black) harlequin opal.

Common Or Fire Opal

Fire opals are transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red and they do not show any play-of-color. The most famous source of fire opals is the state of Queretaro in Mexico and these opals are commonly called Mexican fire opals. Fire opal is strongly characterized by its color, combined with an “amorphous” look, unlike that of transparent crystalline gems.

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