Ten Opal Gemstone Facts (October Stones)
Ten Opal Gemstone Facts
1. Opal is made mainly from silica and water
Opal is a form of hydrated silica and unlike quartz, which has a well-organised crystalline structure, opal is amorphous, meaning it does not have the same regular crystal structure.
One of the surprising things about opal is that it can contain a significant amount of water. GIA states that opal can contain up to around 20% water within its silica structure.
The amount of water varies considerably between different opals, however, so this does not mean that an opal feels wet or contains liquid water like a sponge. The water is incorporated into the material at a microscopic level.
2. The rainbow colors are caused by tiny silica spheres
The spectacular flashes seen in precious opal are known as play-of-color. They are not simply caused by pigments or impurities in the stone. Instead, they result from the microscopic structure of the opal.
Precious opal contains countless tiny silica spheres arranged in a remarkably regular pattern. When light enters the stone, it interacts with these spheres and is diffracted, separating white light into the colors of the spectrum.
The size of the spheres affects which colors are produced. Very small spheres can produce blue and violet flashes, while larger spheres can produce colors towards the orange and red end of the spectrum. Red flashes are particularly uncommon because they require relatively large silica spheres.
This is why an opal can appear to change color as it is moved in the light.
3. Not all opals have rainbow colors
It is easy to assume that every opal should have a colorful display, but that is not the case. Gemologists generally divide opals into two broad groups: precious opal, which displays play-of-color, and common opal, which does not.
Common opal can occur in many colors, including white, pink, blue, green, yellow and brown. Because it lacks the characteristic play-of-color, it can look very different from the colorful opals most people associate with the gemstone.
So, technically, an attractive piece of opal does not necessarily need to flash with rainbow colors to be a genuine opal.
4. There are several very different types of gem opal
Opal is not just one type of gemstone. There are several important varieties, classified partly according to their body color and transparency.
White or light opal has a pale background, while black opal has a dark body color that can provide a dramatic contrast with its play-of-color. Fire opal is usually transparent to translucent and has a yellow, orange or red body color. Boulder opal contains natural fragments of its host rock, usually ironstone, which remain attached to the back of the finished gemstone. Crystal opal is transparent or semi-transparent and can have colors that seem to float within the stone.
This variety is one of the reasons opal can look so different from one gemstone to another.
5. Black opal is not necessarily black all the way through
The term black opal can be slightly misleading. It does not necessarily mean that the entire gemstone is solid black.
The classification refers primarily to the darker body color of the opal. The dark background can make flashes of red, green, blue and other colors appear particularly vivid. Black opals from Lightning Ridge in New South Wales, Australia, are especially famous.
Black opal is generally considered the rarest of the major precious-opal types, which helps explain its reputation among collectors and jewelry enthusiasts.
6. Boulder opal can contain the rock it formed in
Boulder opal is particularly interesting because the natural host rock becomes part of the finished gemstone.
In Australian boulder opal, thin seams of opal can form within ironstone or other host rock. Instead of cutting all of the surrounding rock away, cutters often leave it attached to the back of the opal. The result is a gemstone with a colorful layer of opal contrasting with its natural dark or brownish backing.
This means that two pieces of rough boulder opal can look completely different, depending on the shape and distribution of the opal within the surrounding rock.
7. Australia is famous for opal — and has an extraordinary fossil record
Australia has been one of the world's most important sources of opal for more than a century. Major Australian opal-producing areas include Lightning Ridge in New South Wales and Coober Pedy in South Australia.
But Australia's opal deposits have produced something even more unusual: opals formed from fossils.
Plants and animals can sometimes be replaced by opal during geological processes, preserving remarkably detailed structures. Australia is the only known place where opalised animal and plant fossils have been found in significant numbers. These include marine creatures and prehistoric vertebrates.
One particularly famous example is Eric the Pliosaur, an opalised prehistoric marine reptile discovered near Coober Pedy. The specimen is now part of the Australian Museum's collection.
8. Some opals can actually absorb water
Certain opals are known as hydrophane opals. The name comes from the fact that they are porous enough to absorb water.
Ethiopian opals are particularly well known for this property. When a hydrophane opal absorbs water, its appearance can temporarily change and its weight can increase slightly. GIA has even examined strands of natural opal beads that absorbed drops of water during laboratory testing.
This is more than just an interesting scientific curiosity. Because hydrophane opal can be porous, prolonged exposure to water can sometimes create durability concerns, and some stones can be more vulnerable to cracking.
9. Opal is relatively soft compared with many popular gemstones
Opal has a hardness of approximately 5 to 6.5 on the Mohs scale, depending on the material and source.
For comparison, diamond measures 10, sapphire and ruby measure 9, and quartz measures 7. This means that opal can be scratched more easily than many of the gemstones commonly used in jewelry.
Opal can also be sensitive to sudden changes in temperature and drying conditions. This is one reason opal jewelry needs a little more care than particularly hard gemstones such as sapphire.
10. No two opals are exactly alike
Perhaps the most fascinating thing about opal is its individuality. The microscopic arrangement of silica spheres varies from stone to stone, producing different colors, patterns and areas of play-of-color.
Some opals show broad flashes of color, while others display small pinpoints, rolling patches or intricate patterns. The colors can also appear to change depending on the viewing angle and the lighting.
This makes choosing an opal rather different from choosing many other gemstones. With a sapphire, for example, color, clarity and cut can be evaluated using relatively standard criteria. With opal, the particular pattern and distribution of its play-of-color are a major part of what makes an individual stone attractive and valuable.
It is this extraordinary variation that has helped make opal one of the world's most recognisable gemstones. From the dark colors of Australian black opal to the fiery orange body color of Mexican fire opal and the transparent appearance of crystal opal, there is an enormous range of natural appearances to discover.
October Gemstones ~ Opal Quiz
As you may know Opal is one of the gemstones of October together with Tourmaline which are both multi-colored. Here is a fun trivia quiz about this interesting gemstone.
No email is required - your score and the answers will come up here after you hit the Submit button :).
