Jeweller - October 2021

REVIEW
Gems
Unusual Opals Part II: Boulder & Matrix Opal
L to R: Linneys earrings; Opal Minded
necklace; Cartier bracelet Below:
Cartier ring; Katherine Jetter ring
Australia – the home of opal – is well known
the world over for black and white opal
specimens; however, boulder and matrix
opals are a huge part of the Australian opal
industry and only increasing in popularity.
So, what’s the difference between them?
The classification of natural opal depends
on the relationship between the opal itself
and the host rock in which it forms.
The more well-known black and white
opals are termed ‘natural type 1’
and feature solely the opal itself, of a
reasonably homogenous composition.
In contrast, boulder opal is ‘natural type
2’, a single piece of opal that remains
naturally attached to its host rock.
Matrix opal, on the other hand, is ‘natural
type 3’, in which opal snakes through
pores and grains and/or fills holes within
the host rock – think 'in' rock, as opposed
to boulder opal being 'on' rock.
Generally, natural type 1 opal is the most
valuable, followed by natural type 2, then
natural type 3.
Boulder and matrix opal are both a type
of sedimentary opal.
The host rock depends on where it
comes from – sandstone or ironstone
for Queensland opal specimens, and
quartzite, sandstone, or claystone for
those from Andamooka, South Australia.
The formation of opal is a complex
curiosity, highly debated among geologists
and scientists.
There have been many hypotheses
produced over the years, with most
controversy and debate happening in
more recent times.
What is agreed is the necessity of certain
factors for the formation of opal – a source
of silica, availability of water, the right
chemical and geological conditions, and
adequate time.
Most of the major opal-producing fields
across Australia have a similar geological
setting, consisting of a thick layer of
sandstone atop a layer of clay.
The most generally accepted hypothesis
of opal formation is known as the ‘deep
weathering theory’, in which water
flows through the top sandstone layer,
picking up silica and continuing down
through cracks and crevices to the
bottom clay layer.
There, the now silica-rich solution is
deposited, forming opal.
The well-known ‘Yowah nut’ opals from
Queensland are believed to form in this
way. Yowah nuts are small boulder opals
that resemble tree nuts, with a precious
or common opal centre surrounded by an
outer layer of ironstone.
All Queensland boulder opal and matrix
opal from Andamooka formed during
the Cretaceous period, 60–144 million
years ago.
Andamooka matrix opal is known for being
treated to appear darker through a simple
process involving infusing the opal with a
Boulder &
Matrix Opal
Named for the way the
opal forms and enjoins
the host rock
Colour: Multiple
Found in: Australia,
Honduras, Mexico
Mohs Hardness: 5–6.5
Class: Silicate
Lustre: Subvitreous
Formula: SiO 2
.nH 2
O
sugar-rich solution, followed by boiling in
concentrated sulphuric acid.
This results in carbonised sugar
throughout the matrix which turns the
specimen black and brilliantly highlights
play-of-colour, imitating black opal.
Other methods of producing a similar
result have also been reported.
An interesting opal triplet imitating natural
boulder opal has also been noted. Triplets
are a type of composite opal involving a
thin piece of natural opal between a dark
backing and domed quartz or glass.
In this instance, a piece of crystal opal sits
atop a black layer, followed by an unevenly
joined piece of what is believed to be a
resin and ground boulder matrix mix.
The fourth and final layer is a solid
piece of natural boulder matrix, with
veins of precious opal. According to the
manufacturer, these imitants are made in
Hong Kong using Coober Pedy crystal opal
and Queensland boulder 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
October 2021 | 29