The first discovery was in the Menz Gishe District of Shewa Province.
Opal from this area occurs in a wide range of body colours, including brown, red, orange, yellow, and white.
Often marketed as 'Shewa opal' or 'Mezezo opal', these opals form in stratified igneous rocks such as rhyolite, tuff, and ignimbrite.
However, perhaps the most important Ethiopian opal discovery occurred in 2008, with a deposit of hydrophane material located near the town of Wegel Tena in Wollo Province.
The find consisted 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.
This opal is often known as 'Welo opal' or simply 'Ethiopian opal'. This region produces hydrophane opal that is usually opaque to translucent in white, brown, orange, and colourless body-colours.
Specimens are also known to display strong play-of-colour in some specimens and be similar looking to non-hydrophane material from Brazil and Australia.
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.
Like all opal, hydrophane opal is hydrated silica – but 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.
The absorption by hydrophane opal is considerable and must be accounted for when handling and storing.
Common sense would tell us that material capable of absorbing liquid and changing so easily should be treated with caution. The issue here is the fact that hydrophane is often not disclosed at the time of transaction.
Given the tendency to change appearance when immersed, avoid ultrasonic and steam cleaning of this opal variety – wipe over with a soft cloth instead.
Other precautions 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.
To test the presence of this feature in an opal, bring the opal in contact with a single drop of water while observing it with a hand lens. Watch how the water interacts with the gemstone, before testing the refractive index of the area.
Be patient with the test; you may need to wait four to five minutes or so to see the result. 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 colours, including purple.
A primitive and effective form of treatment documented is smoke treatment, commonly applied to hydrophane specimens in more recent times.
By wrapping gemstones in material such as newspaper or bark, followed by aluminium foil, and placing them into a burning fire, the material is carbonised and produces a dark body-colour throughout the stone, resembling valuable black opal.
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