Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Sophia Monge
The History of Opal
The earliest known opal that was discovered dated back to around
4000 B.C. The opal artifact were said to be found in Kenya by an anthropologist
named Louis Leaky. The opals were discovered in a similar way any other precious
gemstones were found, in a cave mining process, and were slowly practiced on to
create different shapes for things such as jewelry. The most popular opals are
found in Australia, Mexico, and the U.S.
What is it?
The opal, however, is not a true gemstone or a mineral, rather it
is commonly known as a mineraloid. The way to pick out an opal is by its color.
The most common colors could include white, yellow, orange, purple, blue, green,
grey, black, and brown. The colors, however, could change when looked at in
different lighting and different angles. On the scale of hardness, the opal come
in at around 4.5-6.5, according to the minerals.net website. The composition of
an opal is “Hydrous silicon
dioxide. The water can range from 3% to 21% of the total weight, but is usually
between 6% to 10%” (minerals.net) and the chemical formula is SiO2 · nH2O
Weller/ Cochise College
Though there are many different types of opals in the world, such
as the fire opal, Idaho star opal, Arizona blue opal, and the Mexican jelly
opal, the Australian black opal is the most valued opal. It is very rare to
find, very rare in color and “have
a dark background and colours ranging from brilliant red through to greens,
blues and purples.” (Australian Mines Atlas). The name black opal comes from
the definition of the opal having a black or grey base color; a lot of people
mistake the black opal as being just that, a black tone based opal.
were lucky enough to find a black opal with one grey-based side and one
black-based side, it would be called a reversible opal. The first black opal was
found in 1873 however, people were not aware of how much a black opal was worth
until 1903. During that time the black opal was very valuable and the net worth
of the opal only increased as time went on. According to the opalauctions.com
website, the black opals were always the buyers top choice and the price could
increase up to 20% per year. Some black opals value could range between
$1,000-$3,000 per carat.
R. Weller/ Cochise College
How it is Valued
most opals are valued now could include the following; Opal class, body tone,
play of colors brilliance, pattern, color bar, and faults. Though not all opals
go through this specific checklist because of their uniqueness, a lot of the
previous factors are taken into consideration when valuing each opal. The most
important factor though, is the body tone. The body tone determines what color
it is and what kind of opal it is. It could be a black opal or even a light opal
and generally the opals that have a darker undertone are higher valued opals
than ones with the lighter undertone due to the darkness showing off the
different colors better in the light.
Above is a general chart that can be used when determining the value of an opal and its color base and body tone within the opal.
How Jewelry is Made
most common use for the opal is for jewelry and accessories. Having two ring,
earrings, and a necklace with opals in them, I value them because I know how
precious opals can be. In the jewelry most opals are solid stone but others are
not. Some of the not so solid opals have very fine but brilliant layers. The way
those opals are handled are by gluing the fine layer to its base of whatever
jewelry the opal is going into, and then it has a cap put over it protect it
from any wear and tear and damage to the opal itself. There are three ways to
set an opal into jewelry; the solid opal, which has no covering on it, the opal
doublet which is a thin layer of the opal glued onto a base, and the opal
triplet with is a thin layer of the opal glued to the base and covered with a
clear gap. An opal doublet and an opal triplet are worth a lot less than a solid
opal would be worth.
Above are pictures of my own opal rings. These rings value between $80 and
$115. The oval shaped ring is more of a doublet because the opal itself is very
thin. I am not exactly sure how much the heart shaped ring is because it was a
gift. It is a decent shaped opal but it is also very light in color while the
oval shaped one is more of a blue color.
For the month of October the birthstone happens to be the opal. According to the red C jewels website the meaning of the opal is “purity, innocence, and hope. The opal has metaphysical properties of confidence, loyalty, faithfulness, and happiness”. The name “Opal” could also mean “’To see the change in color’”. (Birthday gems.org)
Jelly Opals R. Weller/Cochise College
Synthetic Opal R. Weller/ Cochise College
Blue Opal. R. Weller/ Cochise College