what is the opal glass is all about?

Question

defination, processing, properties, products, types,

Answer 1

Opal glass is glass that resembles an opal, being translucent and white, with a grayish or bluish tint. I believe it is also called milk glass.

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Answer 2

OPAL (Hydrated Silica Glass)
Chemistry: SiO2 - nH2O; Hydrated Silicon Dioxide.
Class: Mineraloids
Group: Some minerologists place Opal in the Quartz Group.
Uses: As a gemstone and ornamental stone.

PHYSICAL CHARACTERISTICS:
Color is white, colorless, pale yellow, pale red, gray or black when impurities are common. Diffraction can cause flashes of any color of the rainbow (opalescent).
Luster is vitreous to pearly.
Transparency: Specimens are transparent to translucent.
Crystal System: Does not apply because opal is amorphous.
Habits include massive, cavity-fillings such as in fractures and geodes, nodular, reniform or as a replacement of other minerals and wood.
Cleavage is absent.
Fracture is conchoidal.
Hardness is 5.5 - 6
Specific Gravity is approximately 2 - 2.5 (light)
Streak is white.
Other Characteristics: Most specimens will fluoresce white or pale green, some phosphoresce and all specimens can be very sensitive to impacts and low temperatures.
Associated rocks are chert (a form of microcrystalline quartz), volcanic rocks and many others.
Notable Occurrences include many Western USA localities; Mexico; Australia; England; Czech Republic and many other localities around the world.
Best Field Indicators are color play (opalescence), low density, fluorescence, fracture filling tendency and lack of cleavage or crystal faces.

Opal has been a popular gem for many centuries and has a very interesting structure. Opal is considered a mineraloid because this structure is not truly crystalline. The chemistry of Opal is primarily SiO2 and varying amounts of water. The amount of water varies from 5 -10% and greater. This water can help geologists determine the temperature of the host rock at the time the opal formed.
Although there is no crystal structure, (meaning a regular arrangement of atoms) opal does possess a structure nonetheless. Random chains of silicon and oxygen are packed into extraordinarily tiny spheres. These spheres in most Opals are irregular in size and inconsistent in concentration. Yet in Precious Opal, the variety used most often in jewelry, there are many organized pockets of the spheres. These pockets contain spheres of approximately equal size and have a regular concentration, or structure, of the spheres. This has the effect of diffracting light at various wavelengths, creating colors. Each pocket produces a different color, with a different intensity depending on the angle from which a viewer sees it. The multicolored flashes of light that Opal emits gives it a truly beautiful and valuable look.

The name opal probably is derived from the Sanskrit name for precious stone; upala. 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. Today most precious opal comes from Australia with significant sources from Mexico and the Western United States.

Not all opal is so precious however. Common opal lacks opalescence, color or luster and is . . . after all . . . common. Opal is often imitated, forged and "enhanced". Fluorescence, while somewhat unreliable is a good method to determine authenticity.