Opal is a mineraloid gel which is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock , being most commonly found with limonite , sandstone , rhyolite , and basalt . The water content is usually between three and ten percent, but can be as high as 20%. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. The word opal comes from the Latin opalus . Opal is one of the mineraloids that can form or replace fossils . Opals are also Australia 's national gemstone.
At the micro scale precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic closed-packed lattice . These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of opal. It is the regularity of the sizes of the spheres, and of the packing of these spheres that determines the quality of precious opal.
Physical character:
Category
Mineraloid
Chemical formula
Hydrated silica
Color
White, black, red, orange, colorless, iridescent
Crystal habit
Irregular veins, in masses, in nodules
Crystal system
Amorphous
Cleavage
None
Fracture
Conchoidal to uneven
Mohs’ Scale hardness
5.5–6.5
Luster
Subvitreous to waxy
Polish luster
Vitreous to resinous
Refractive index
1.450 (+0.020, -0.080)
Mexican opal may read as low as 1.37, but typically reads 1.42–1.43
Single refractive, often anomalous double refractive due to strain
Birefringence
none
Pleochroism
None
Ultraviolet fluorescence
Black or white body color: inert to white to moderate light blue, green, or yellow in long and short wave. May also phosphoresce.
Fire opal : inert to moderate greenish brown in long and short wave, may phosphoresce.
Absorption spectra
green stones: 660nm, 470nm cutoff
Streak
White
Specific gravity
2.15 (+0.08, -0.90)
Diagnostic Features
darkening upon heating
Solubility
hot saltwater , methanol , humic acid , hydrofluoric acid
Diaphaneity
opaque, translucent, transparent
Some varieties of opal
Fire opal : fire opals are transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red. The most famous source of fire opals is the state of Queretaro in Mexico and these opals are commonly called Mexican fire opals. Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.
Main source of opal
Australia produces around 97% of the world’s opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% but not 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.
News:
In late 2008, NASA announced that it had discovered opal deposits on Mars .
Synthetic opal
As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974. The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous; some may even stick to the tongue.
Two notable producers of synthetic opal are the companies Inamori and Kyocera of Japan . Most so-called synthetics, however, are more correctly termed imitations , as they contain substances not found in natural opal The imitation opals seen in vintage jewellery are often "Slocum Stone" consisting of laminated glass with bits of foil interspersed.
Historical superstitions
Opal is the traditional birthstone of the month of October.
Some famous opals
1.The Andamooka Opal : presented to Queen Elizabeth II , also known as the Queen's Opal
2.The Aurora Australis Opal : considered to be the most valuable black opal
3.The Black Prince Opal : originally known as Harlequin Prince
4.The Empress of Australia Opal
5.The Halley's Comet Opal : the world's largest uncut black opal
6.The Olympic Australis Opal : reported to be the largest and most valuable uncut gem opal ever found
7.The Pride of Australia Opal : also known as the Red Emperor Opal
8.The Red Admiral Opal : also known as the Butterfly Stone
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