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 Home / Gem Library / Natural Gemstones / Opal July 18, 2024  

Natural Opal
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There are 28 Opal(s) available at

Opal is a poorly crystalline or amorphous hydrous silica that is compact and vitreous and most commonly translucent white to colourless. Precious opal reflects light with a play of brilliant colours across the visible spectrum, red being the most valued. Opal forms by precipitation from silica-bearing solutions near the Earth's surface. Electron microscopy has shown that many opals are composed of spheres of tens to a few thousand angstroms in size that are arranged in either hexagonal or cubic close packing. The spheres are composed of hydrous silica that may be either almost cristobalite-like, tridymite-like, mixtures of both, or random and nondiffracting. The specific gravity and refractive index are lower than those of pure silica minerals. The play of colours in precious opal arises from the diffraction of light from submicroscopic layers of regularly oriented silica spheres. When heated, opal may lose as much as 20 percent of its weight of water, fracture, and then crystallize to one of the silica minerals described above.

Opal usually contains 4 to 9 percent water, but lower and much higher values have been observed. The contents of alumina, ferric oxide, and alkalis are variable but may amount to several percent in light-coloured opals and more if pigmenting minerals are also present. Precious opal has been synthesized. Opaline silica is a friable hydrous silica found near hot springs and geysers.

Known Facts
Color Key: White, green, blue, black, red, orange, violet, pink, grey, yellow.
Refractive Index: 1.44-1.46
Chemical Composition: SIO2nH2O
Hardness: 5.5 - 6.0
Density: 2.65 - 3.00
Crystal Group: Amorphous
Ocurrence: Czechoslovakia, Mexico, Honduras, Australia, Brazil, Tanzania.

Depending on the origin of the opal, the durability and stability of the material is variable. Mexican, Brazilian, and Australian opals appear to be quite stable. Chocolate colored Ethiopean opals from the Mezezo deposit are quite attractive but crack easily as a result of even the slightest temperature variations.

Wegel Tena Opal

The most important and exciting new discovery of opal is also in Ethiopia, but at the deposit near Wegel Tena, 200km north of Mezezo and 550km north of Addis Abba.

Opal from this new area has been mined since 2008. The entire region around Wegel Tena consists of a volcano-sedimentary sequence with alternating layers of basalt and rhyolitic ignimbrite. Over the entire region, only one thin seam is mineralized with opal. The opal is extracted by artisanal miners using picks, hammers, and shovels.

The body color is typically white with some brown and play of color. Unlike the material from Mezezo this material is remarkably tough and resitant to crazing.

Perhaps the most unusual feature of the material is its hydrophane (water absorbing) character. When soaked in water, many of the stones become transparent. However, the phenomena is completely is reversible and repeatable and all the color will return upon drying although drying may take a few days. The absorption of water is related to the opal's porosity and as a result, the specific gravity and weights of the gems may vary by as much as 10% upon immersion in water.

Some of the stones display pinpoint play of color as well as an interior lizard skin or digit pattern which differentiates these opals from other opals. Because of the unique characteristics, even experts have confused this material with synthetic opal.

Although a thorough geological exploration will be required to determine the extent of this deposit, it is already the most significant discovery of high quality opal in many years.

Opal Opal Opal
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Opal Stats/
Popularity Index: 1400812hits
Total Items: 28items
Total Weight: CTS 334.33
Cheapest: USD 36.00
Most Expensive: USD 1402.20
Largest: CTS 58.36
Smallest: CTS 3.69
Last Added: 10/31/23 09:26:17

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