General Information about Opal
- Opal is a mineraloid composed of hydrated silica.
- It has a hardness of 5.5-6 on the Mohs scale.
- Opal can have a luster ranging from subvitreous to waxy.
- The streak of opal is white.
- It can be opaque, translucent, or transparent.
Formation and Composition of Opal
- Opal is deposited at relatively low temperatures.
- It can occur in the fissures of various types of rocks.
- Opal is commonly found with limonite, sandstone, rhyolite, marl, and basalt.
- Its water content can range from 3 to 21% by weight.
- Opal is classified as a mineraloid due to its amorphous property.
Opal's Optical Properties
- Opal is single refractive but can exhibit anomalous double refraction due to strain.
- The refractive index of opal can vary, with Mexican opal typically ranging from 1.42-1.43.
- Precious opal displays play-of-color, while common opal does not.
- Play-of-color is a pseudo chromatic optical effect resulting in flashes of colored light.
- Opal's internal structure causes it to diffract light, resulting in play-of-color.
Precious Opal
- Precious opal consists of silica spheres arranged in regular, closely packed planes.
- These spheres, around 150-300nm in diameter, create the play-of-color effect.
- The quality of precious opal depends on the regularity and packing of these spheres.
- Precious opal can be transparent, translucent, or opaque.
- Black opal is considered the rarest, while white, gray, and green opals are more common.
Opal Sources
- Australia and Ethiopia are the primary sources of opal.
- Australian opal accounts for 95-97% of the world's supply.
- South Australia alone accounts for 80% of the world's supply.
- In 2012, Ethiopian opal production was estimated to be 14,000kg.
- Australian opal production in 2012 was valued at $41 million.
Opal Mentions
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Opal Data Sources
| Reference | URL |
|---|---|
| Glossary | https://harryandcojewellery.com.au/blogs/glossary/opal |
| Wikipedia | http://en.wikipedia.org/wiki/Opal |
| Wikidata | https://www.wikidata.org/wiki/Q171058 |
| Knowledge Graph | https://www.google.com/search?kgmid=/m/027l73t |