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The Formation & Mining of Coober Pedy Opal

By Carmen Hui

The Formation of Coober Pedy Opal

Structure of Coober Pedy Opal

Opal is an amorphous form of hydrated silica (SiO2·nH2O). The water content is from 3 to 21% by weight. Coober Pedy opal is usually between 6 and 10%, which is comparatively lower water content than Ethiopian Opal. Coober Pedy Opal is also less porous. Due to the low water content of the structure, the risk of dehydration is also lower. Therefore, the stability of Coober Pedy Opal is better.

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Play of Colour

The internal structure of a precious opal enables it to show a vivid play of iridescent colour. This optical effect is called ‘ The Play of Colour’, which is also one of the crucial standards to classify whether it is a precious opal or common opal (potch). 
The play-of-colour in precious opals may exhibit every prismatic colour from violet to red, owning to a combination of diffraction and interference. 
Under the 10x lens, these patches show a satiny lustre with parallel striations which vary in orientation from patch to patch. 
The iridescent colours usually change with the viewing angle. The samples below show the play of colour. The colours change when the stone is viewed with different angles. Therefore, it is quite important to find out the best direction of the play of colour during cutting and fashioning in order to display the best optical effect.
opal samples.jpg
Coober Pedy opal roughs.jpg
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Geoglogy of Coober Pedy Opal

From about 130 to 93 million years ago,the Great Artesian Basin in Australia was flooded. It came to be the Eromanga Sea, which was cold, shallow, large scale and disconnected with the open ocean. 

At the end of the Cretaceous and into the early Palaeogene (from about 93 to 60 million years ago), the sea was slowly uplift and eroded.  After that it started to dry out and acidic weathering happened. The weathering process broke down minerals of rocks into clay and soluble silica. It also made cavities in the rocks and fossil shells. These cavities, together with faults and fractures in the rock, provided pathways for underground water containing the soluble silica. These fluid also trapped clay deposits. As time passed, silica-rich solutions deposited into opal.
In figure 5, there are the opals formed in Australia. All of them are inside the Great Artesian Basin.
opal field.jpg
Conditions for formation
There are six necessary conditions for the formation of opal, mentioned by Dr. Patrice Rey.
  • Quiescent conditions
  • Uniform concentration of silica in the form of the primary colloid particles.
  • Slow and uniform aggregation of these particles.
  • Aggregation to a size suitable for optical diffraction effects to occur when solidified.
  • Slow settling of the spheres under conditions to allow them to pack in a regular fashion.
  • Slow removal of water, in the first place by absorption by underlying clay, and finally by slow drying.
It is obvious that the combination of the above conditions is rare in the world, which explains why there are limited occurrences of the gem quality opal in the world. As irregular growth rates, ground movement, unstable concentration of silica and other factors would affect the size and structure of the spheres formed. 
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white opal types.jpg

Types of Coober Pedy opal

In fact, not all the Coober Pedy opals can be distinguished from other opal as all the quality of opals found varies from potch to precious opals.
Body colour

Most of the precious opals found in Coober Pedy are ‘White opal’, which has a white body colour with the play of colour. It can be referred as the crystal type (translucent) (Fig.6) and the milky type (opaque) (Fig.7).  
Coober Pedy opal roughs contain various skins and sand. By observing the surface, the opal expert can know the quality of the opal and which field it comes from.
opal roughs with various skin.jpg
opalised belemnite fossil.jpg
Opalized fossil
Some Coober Pedy opals are pseudomorph. They were formed after the host rock stopped growing and altered or replaced by silica. There are more opalized fossils from shells to bones compared with other opal fields.
Opal belemnite fossil (Fig.9) are also called pipe opal. It is found in pipe-like structures which may be hollow or opal-filled. The sample in figure 9 is partly opal-filled and hollow at the end. It is translucent with the play of colour.
The shell surface textures and structures are clearly seen on the opalised bivalves (Fig.10) and univalves (Fig.11).
For opalised bivalves, there is usually sand between the opalised shells. It is more often found in half of the shell, either the top or the bottom of the bivalves where the curved rough can be recognised as opalised bivalves. 
For opalised univalves, the gastropods with whorl structure can been seen.
These are all extremely rare opalization forms on the fossil (Fig12). Today only collectors or museums have these incredible specimens.
opalised fossil.jpg
solid opal size &ff.jpg
Cutting of Coober Pedy opal
Most of them are cut in cabochons in order to show its play of colour and irregular iridescent colour patches. Some of them are also cut in free form so as to enhance the yield.
opal cameo.jpg
Some of them are cut as sculpture called opal cameos. The shape and design are based on the the opal rough found.
Besides solid opal, Coober Pedy opal is chosen to be cut in opal doublet or triplet as their fire of the play of colour is strong and firm even when cut into slice. It is sliced into a very thin piece and put on a dark background, which enhances its brilliant colours. Opal doublet simulates boulder opal, and opal triplet usually simulates black opal. 
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opal doublet & triplet.jpg

Micro-to Nano Structure

Under scanning electron microscope, the structure of opal is disclosed. It shows a 3D regularly stacked group of silica spheres. The iridescence depends on the uniformity of size and arrangement of these spheres.
Light is reflected by different silica sphere layers. The reflected rays interfere and enhance the visibility of certain wavelength (or color). The process is called diffraction.
The diffracted color depends on the spacing between silica spheres, the orientation of crystal layers and viewing angle. The silica lattice structure must consists of spheres with a uniform diameter between 200 - 350 nm to be able to produce a play-of-colour in the visible light range from violet to red (wavelength 400 - 700 nm).

To undergo SEM inspection, the samples have to be electrically conductive and hence, are coated a thin layer of gold (about 40nm thick) using gold sputtering machine. Then, the testing sample was put into the stage of SEM.
HF etching.jpg

*private connection with SEM Laboratory

Without chemical treatment, no silica sphere can be observed in the SEM image (Figure 18, left). This is because cement is sticking with the silica spheres and filling the spacing between spheres. After we used 10% hydrofluoric acid (HF) for etching process, the typical structure of precious opal with play of colour was presented. The cement was in this case completely dissolved by HF and the spheres was exposed.
SEM opal testing.jpg
For diffraction to occur, it can be described by Bragg's law,
n λ = 2 d sin(α)
α = angle between the diffracted waves and the lattice plane
λ = wavelength of light
d = diameter of the silica spheres
n = 1, 2, 3…
opal sphere structure.jpg
Gaps between spheres are of the same order of size as wavelengths of visible light. Some gaps are large enough to allow all visible light rays to be refracted, diffracted and reflected, which
produce colour from red to violet (such as sample II in figure 17). Other smaller gaps only allow green to violet light so they only produce green to violet colours. Smaller gaps allow a narrower range of wavelength (or color) of light to be diffracted. For example, sample I in figure 17 exhibits the vivid color of blue to violet.

*private connection with SEM Laboratory

For sample I, the distance between 4 spheres are 967.4nm
by Bragg's law,
n λ = 2 d sin(α)
(1)λ = 2 (967.4/4) sin (90) , assume n=1; α=90

λ = 483.7nm
This is the wavelength of blue in visible light
n λ = 2 d sin(α)
(1)λ = 2 (967.4/4) sin (60) , assume n=1; α=60

λ = 418.9nm
This is the wavelength of violet in visible light
By varying the incident angle, the spheres of sample I produce a play of colour in the visible light range from blue to violet.
SEM image red opal.jpg

*private connection with SEM Laboratory

For sample II, the distance between 8 spheres are 2.604um (2604nm)

by Bragg's law,
n λ = 2 d sin(α)
(1)λ = 2 (2604/8) sin (90) , assume n=1; α=90

λ = 651nm
This is the wavelength of red in visible light

n λ = 2 d sin(α)
(1)λ = 2 (2604/8) sin (45) , assume n=1; α=45

λ = 554nm
This is the wavelength of green in visible light

n λ = 2 d sin(α)
(1)λ = 2 (2604/8) sin (30) , assume n=1; α=45

λ = 325.5nm
This is the wavelength of ultraviolet

It shows when the incident angle is changed, the wavelength is also changed. By varying the incident angle, the spheres of sample II produce a play of colour in the visible light range from red to violet.
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