Hydrophane opal is hydrated silica, like all opal, with a unique characteristic – a level of porosity allowing water and other liquids to seep in and change the colour, and even the weight, of the gemstone.
Although this porous nature occurs in all opal, the absorption by hydrophane opal is considerable and must be accounted for when handling and storing.
Hydrophane opal is more commonly known in the industry as Welo opal or Ethiopian opal, derived from the Wollo province in Ethiopia – an especially well-known source.
The specimens of this region tend to produce hydrophane opal that is opaque to translucent in white, brown, orange, and colourless body-colours.
They are also known to display strong play-of-colour and offer a more cost-effective option to similar looking, non-hydrophane material from Brazil and Australia.
Hydrophane opal of Ethiopia is a type of volcanic opal, binding volcanic debris together by forming in the gaps between them.
A deposit near the town of Wegal Tena in northern Ethiopia consists of a single seam of opal, less than 1m thick, sitting within a rocky cliff overlooking a canyon – an example of the terrain miners must navigate to retrieve these gemstones.
To make things even trickier, mining is often carried out with simple hand tools and limited safety considerations.
A notable feature of these Ethiopian opals is the digit pattern – a captivating pattern across the gem of rounded columns said to resemble fingers.
The pattern is so well known, it is thought of as an identifying, though inconclusive, feature of Ethiopian material.
Other hydrophane producing areas include Indonesia and the Virgin Valley opal field in the US.
Common sense would tell us that material capable of absorbing liquid and changing so easily should be treated with caution.
Depending on the condition of the gemstone to begin with, hydrophane opal may be less durable than other types of opal and susceptible to cracking and crazing, particularly if immersed.
Therefore, avoid ultrasonic and steam cleaning – wipe over with a soft cloth instead.
Other precautions that are easily overlooked include avoiding perfumes, hairspray, oils, cleaning agents, and any other liquids.
Much like pearls, the best rule of thumb is to put hydrophane opal jewellery on last and take it off first.
Being aware of the opal’s hydrophane nature is important to ensure you can pass on proper care instructions to your client. To test the presence of this feature in an opal, bring the opal in contact with a single drop of water while observing it under the microscope.
Watch how the water interacts with the gemstone, before testing the refractive index of the area.
If hydrophane material, the tested area will have a slightly different refractive index to the rest of the stone – a result of the absorption.
This characteristic absorption property makes them susceptible to being treated with dyes to change the body colour.
Gemmologists and buyers should be cautious of treatment in hydrophane opal, particularly in gemstones with natural-looking body-colours other than white.
Treated gemstones have been documented in all kinds of interesting colours, including purple.
A primitive and effective form of treatment documented in history is smoke treatment, applied to hydrophane specimens in more recent times.
By wrapping gemstones in material such as newspaper or bark and placing them into a burning fire, the material is carbonised and produces a dark body-colour throughout the stone, resembling valuable black opal.