Jeweller - November 2021

REVIEW
Gems
Unusual Opals Part III: Ethiopian Opal
L to R: Kimberly McDonald earrings;
Boucheron necklace; Chaumet
bracelet Below: Cartier necklace;
Ornella Ianuzzi ring
While Australia remains the world's
premier supplier of opal – accounting for
approximately 90 per cent of the opal on
the market – significant opal deposits were
discovered in Ethiopia in 1994, 2008 and 2013.
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
Ethiopian Opal
From the country in
which it is found
Colour: Multiple
Found in: Ethiopia
Mohs Hardness: 5–6.5
Class: Silicate
Lustre: Subvitreous
Formula: SiO 2
.nH 2
O
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 naturallooking
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.
Mikaelah Egan FGAA Dip DT
began her career in the industry at
Diamonds of Distinction in 2015. She now
balances her role as a gemmologist at
Vault Valuations in Brisbane with studying
geology at the University of Queensland.
Visit instagram.com/mikaelah.egan
For more information on gems and
gemmology ,go to www.gem.org.au
November 2021 | 33