Opal

Opal


Published On: 10-31-2013 06:48pm

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Category: Gemstones

Opal has always been fascinating to me.  The iridescence is so striking.  What I didn’t find out until recently though is that is occurs in many different colors that the white one that I saw growing up.  Opal is also the birthstone for the month of October.  Here is some great information I found when researching this favorite.


Opal is a hydrated amorphous form of silica; its water content may range from 3% to 21% by weight, but is usually between 6% to 10%. Because of its amorphous character it is classed as a mineraloid, unlike the other crystalline forms of silica which are classed as minerals. It 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, marl and basalt. Opal is the national gemstone of Australia, which produces 97% of the world's supply. This includes the production of the state of South Australia, which amounts to around 80% of the world's supply.

The internal structure of precious opal makes it diffract light; depending on the conditions in which it formed, it can take on many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. It varies in optical density from opaque to semi-transparent.



Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality); resin opal, which is honey-yellow with a resinous luster; wood opal, which is caused by the replacement of the organic material in wood with opal; menilite, which is brown or grey; hyalite, a colorless glass-clear opal sometimes called Muller's Glass; geyserite, also called siliceous sinter, deposited around hot springs or geysers; and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.

More rare are the Fire opals which are transparent to translucent opals with warm body colors of yellow, orange, orange-yellow or red. They do not usually show any play of color, although occasionally a stone will exhibit bright green flashes. The most famous source of fire opals is the state of Querétaro in Mexico; these opals are commonly called Mexican fire opals. Fire opals that do not show play of color are sometimes referred to as jelly opals. There is also a type of opal from Mexico referred to as Mexican Water



Opal, which is a colorless opal which exhibits either a bluish or golden internal sheen.

Girasol opal is a term sometimes mistakenly and improperly used to refer to fire opals as well as a type of transparent to semi-transparent type milky quartz from Madagascar which displays an asterism, or star effect, when cut properly. However, there is a true girasol opal that is a type of halite opal, that exhibits a bluish glow or sheen that follows the light source around. It is not a play of color as seen in precious opal but rather an effect from microscopic inclusions. It is also sometimes referred to as water opal as well when it is from Mexico. The two most notable locations of this type of opal are Oregon and Mexico.

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. Blue opal also comes from Oregon in the Owhyee region as well as from Nevada around Virgin Valley.

In late 2008, NASA announced that it had discovered opal deposits on Mars.

 

Sources:   http://en.wikipedia.org/wiki/Opal


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