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 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.
