Opal has always been fascinating to me.Â The iridescence is so striking.Â What I didnâ€™t find out until recently though
is that is occurs in many different colors that the white one that I saw
growing up.Â Opal is also the birthstone for the month of October.Â 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,
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
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 diatomaceous earth, the accumulations of diatom shells
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 QuerÃ©taro
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:Â Â http://en.wikipedia.org/wiki/Opal