Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 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.
There are numerous types of opal, some of which are;
Peruvian opal (also called blue opal) is a semiopaque 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.
Blue opal also comes from Oregon in the Owhyee region, as well as from Nevada around Virgin Valley.
Girasol opal is a term sometimes
mistakenly and improperly used to refer to fire opals, as well as a type of transparent to semitransparent type milky quartz from Madagascar which displays
an asterism, or star effect, when cut properly. However, the true girasol opal is a type of hyalite opal that exhibits a bluish glow or sheen that follows the light source around. It is not a play of colour as
seen in precious opal, but rather an effect from microscopic inclusions. It is also sometimes referred to as water opal, too,
when it is from Mexico. The two most notable locations of this type of opal are Oregon and Mexico
Fire opal is a transparent to translucent opal, with warm body colors of yellow to orange to red. Although it does not usually show any play of colour, 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 colour are sometimes referred to as jelly opals. Mexican opals are sometimes cut in their ryholitic host material if it is hard enough to allow cutting and polishing. This type of Mexican opal is referred to as a Cantera
opal. Also, a type of opal from Mexico, referred to as Mexican water opal, is a colourless opal which exhibits either a bluish or golden internal sheen.
Opal is the national gemstone of Australia. Australian opal has often been cited as accounting for 95-97% of the world’s supply of precious opal, with the state of South Australia accounting for
80% of the world’s supply. Recent data suggests that the world supply of precious opal may have changed. In 2012, Ethiopian opal production was estimated to be 14,000 kg (31,000 lb) by the United States Geological Survey.
Other source of white base opal or creamy opal in the United States is
Spencer, Idaho. A high
percentage of the opal found there occurs in thin layers.
Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary,
Turkey, Indonesia, Brazil (in Pedro II, Piauí), Honduras Erandique-
LempiraLempira Department, Guatemala and Nicaragua.
In late 2008, NASA announced it had discovered opal deposits on Mars.
The crystal may not be one of the most popular precious metal on the planet, yet, it commands some importance and value, like
-The Olympic Australis, the world’s
largest and most valuable gem
-The Andamooka Opal, presented to Queen Elizabeth II, also known as the Queen’s Opal
-The Addyman Plesiosaur from
Andamooka, “the finest known opalised skeleton on Earth”
-The Burning of Troy, the now-lost opal presented to Joséphine de Beauharnais by Napoleon I of France and the first named opal
-The Flame Queen Opal
-The Halley’s Comet Opal, the world’s largest uncut black opal
Although the clock faces above the information stand in Grand Central Terminal Manhattan, New York, are often said to be opal, they are in fact opalescent glass
-The Roebling Opal, Smithsonian
-The Galaxy Opal, listed as the “World’s Largest Polished Opal” in the 1992 Guinness Book of Records
SOURCE : WIKIPEDIA