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Article: Opal


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

Opal Data Sources

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