From Opal Butte, Oregon as displayed from Bonhams aucion.
October, the month of colour and spooks. As like the previous months, October has two gemstones associated with it as well. We have the choice of opal or pink tourmaline for October babies and as you can tell from the title, I will be informing you about opals.
Opal is a very interesting gemstone. It is referred to as a hydros amorphous silicate, with the chemical formula SiO2 · nH2O. While it is similar to quartz, it’s the presence of water and the structure of the silica or quartz that’s different. The n refers to a range of water present, roughly 3 to 21% by weight, gem-quality opal is usually 6 to 10 % water weight. Interesting fact, because of its amorphous nature it’s actually referred to as a mineraloid. Amorphous means “without a clearly defined form or shape”. If you were to look at an amorphous solid on the atomic level, you would see nothing lines up in a continuous crystalline structure, it looks more like a liquid. Glass is also an amorphous solid.
Etymologically, opal is derived from several sources. The actual English word “opal” is adapted from the Latin term opalus, meaning “to see a change of colour”. From what I’ve researched it comes from the Sanskrit word úpala which means “precious stone”. Another source refers to the Ancient Greek word from 180 BCE, opallios as a possible source. The Romans adored opal, it was prized beyond all other gemstones as its only source was from Hungary at the time. They marvelled at how it flashed all the colours of the other gemstones. Many cultures believed it was a gemstone that encompassed the spirit of all the other gemstones. Now there are many types, patterns and colours of opal hailing from Australia, Ethiopia, Nevada, Mexico and even Canada. Cool fact, in 2008, NASA announced that they even found opal on Mars.
Opal is identified as:
- 1) Amorphous Crystal System
- 2) Mineraloid Category
- 3) Silicate Family
- 4) Mineral = Hydrous Silicate → Opal -SiO2 · nH2O
- 5) Varieties = Precious Opal, Fire Opal, Potch or Common Opal.
Now it’s not hard to see why opal is so desired, a quick Google image search gives you HOURS worth of images to drool over of this stone. This stone is a phenomenon of physics as light interacts between crystal and water. But wait a minute, didn’t I just say it was amorphous?
Well yes it both is and isn’t. Opal is actually made up of a bunch of tiny quartz spheres that range from 100 – 1000nm in diameter. These spheres can be stacked and arranged into a pattern or random and irregular. Water then fills the gaps between the spheres, aiding in the refraction of light into the quartz. When the spheres and gaps are within 150-300nm in diameter, light is diffracted by the spheres and a prism is produced. It’s similar to a rainbow caused by rainfall, a bunch of little prisms working together to refract white light and create a rainbow. Now when these prism/spheres are in random and irregular order, the light passing through interferes and instead we see the background colour. This is what we call common opal or potch and it can be equally appealing with pink, blue and green. Now, when the spheres are aligned, similar in size and packed together in packages, the light now isn’t interfering and we get precious opal. With precious opal, when you look into the stone the colours you see are directly related to sphere size. Red light = 300nm diameter spheres, while violet = 150nm diameter sphere. Also when the spheres are packing in order, then the spaces in between that are filled with water are also refracting light of the same wavelength, thus the flash is brighter.
Now, due to the variety of colours and patterns within opals, they have their own unique appraisal system. They must look at the body tone, play of colour, pattern type, fire and clarity of the opal. Body tone is the base colour of the opal ranging from black to white. Play of colour is the gemmological term for the colour flash within opal caused by the arrangement of the silica spheres. Side note here, this is not to be confused with another gemmological term opalescence as they have different definitions. Play of colour is how light interacts with the gemstone to produce internal flashing rainbows. Ammolite and fire agate are other gemstones displays “play of colour”. Opalescence is the milky iridescence displayed by an opal. Chalcedony, rose quartz and moonstone will display opalescence as well.
Pattern type is how the play of colour flashes through the opal. Fire is how bright or dull the play of colour is compared to the body colour. Clarity is pretty self-explanatory as it refers to how translucent the opal is. Now, I can spend a whole day explaining the different varieties of opal but that will be for another blog. From my understanding and expertise in gemmology I can tell you one of the most sought-after opals is a black Harlequin opal.
As all gemstones are precious and expensive, opal being a jewel among them, also have imitation and synthetic stones to watch out for. First, we need to discuss how opals are cut and set as opals are softer stones (5.5-6.5 Mohs) that are both brittle and susceptible to temperature changes. Opals come as a “solid”,” doublet” or “triplet”. For ease of understanding let’s start with the doublet. A doublet is when an opal slab is backed with black backing, usually onyx, glass or host rock. This helps protect the opal, and bring the colour out as the black adds to the contrast of the fire and play-of-colour. A triplet is similar to a doublet but the slab of opal is now a thin sheet and a clear cap of either quartz, glass or plastic. Triplets are used when the opal mineralizes in thin sheets within the rock, and there isn’t enough to make a doublet. Finally a solid as its name says a solid opal. Pricing varies as a solid opal is rarer and desired more than a triplet.
When you’re looking for opal jewellery be wary of this as some doublet and triplets look like solid opals. The best indicator is the price. Like with all gemstones, if the price is a really good deal that almost seems too good to be true, it just may now be what you think. Take a close look at the stone and check for a black back or a clear top.
For opal, there are imitations and treatments but no genuine synthetics. This is because the irregularity within is part of the desire and while there are “synthetics” they are more imitations because of the use of either bonding agents like epoxy or other chemicals not found in natural opals. Let’s start with treatments. There are several to be aware of and most of them are to help darken the body colour or increase durability and help with crazing. Crazing is a natural problem with opals as is a result of water loss and temperature changes. The result is fractures throughout your opal that can lead to chipping or breaking the opal. Epoxy, oils and other bonding agents have been used to help seal the fractures and increase the clarity of the opal. This is known as resin treatments and can also be used to help with protecting the surface from damage as well. Another known agent used is aluminum oxide to help cost the opal and help with preventing scratches. This is actually helpful for your opals longevity but it should be indicated with purchase.
Another treatment used for opals is to blacken the back body. Black body colour allows you to see the pay of colour better, hence it’s a higher price and the most common treatment for opals. Several techniques are used for this treatment: carbonizing, acid and sugar, charcoal burning and smoke treatments. Carbonizing involves impacting graphite into the pores and gaps within the white opal matrix. This treatment can be spotted in a microscope with the presents of black clusters or spots. Acid and sugar is a really interesting technique. This involved soaking or “cooking” the cut opal in a saturated sugar solution. Then after some time has passed the opal is removed from the sugar and placed in a concentrated sulfuric acid solution. The acid burns the sugar that soaked into the opal, creating black clusters within the stone. Again, with a microscope, you will see a peppery like texture on the surface and within the opal. Finally, the harcoal and smoke treatment are just as the name states. The opal is wrapped in newspaper or bark and ignited. The ask and smoke are absorbed into the water content of the opal and the stone appears black.
Synthetics and imitation opals are an interesting bunch. For starters, any imitation with a name in front of it or using opal in the name aside from the obvious ones (precious, boulder, water, fire etc…) should be given a look over. There are many glass imitations like Slocum Stone that use polychromatic foil in glass to mimic play-of-colour. Another one I see a lot is opalite. Again this is just opalescent glass, there is no play of colour in the glass.
Synthetic opals on the other hand are made of silica and mimic the amorphous orderly sphere structure. Yet they are not really a true synthetic as many need to use a binding agent like epoxy or glass. These opals can come in a wide range of beautiful colours but limited to patterns. They are usually sold as “lab-grown” opal or they will use their names, such as the case for Gilson opal or Kyocera. The difference is natural opals will display a wider and more random array of colours and patterns. They may have inclusions from the host rock in as well, giving you strong evidence it is a natural opal. Synthetic opals on the other hand will appear spotless and display only two distinct patterns; harlequin and columnar. These patterns are a result of the growth direction from crystallization and with a microscope, you can see what is known as “lizard skin” or “chicken wire” texture. Naturals don’t display this kind of texture.
I have attached a few links for you to look through at your leisure if you are interested in learning more about opals and their patterns.
https://en.wikipedia.org/wiki/Opal
Wikipedia – never to be quoted for a research paper but still a great source of information to start.
https://www.gia.edu/opal-quality-factor
GIA – another great source for gemmological information.
https://www.gemsociety.org/article/opal-jewelry-and-gemstone-information/
IGS – another great information source for gemstones, however, for some articles you will need a membership.
Opal Academy
Hope you have enjoyed my little article about opal. Next month, November, I will be discussing topaz.
Peter