Did you know that Opal is the birthstone of October?  To celebrate this month of opal, we would like to share with you a little information about this amazing gemstone, and why we are so passionate about it here at Giulians!

What is opal, and how is it formed?

Opal is a form of hydrated silicon dioxide (chemical formula SiO₂·nH₂O).  It is formed in dessert-like areas, where the geology is rich in silica deposits – sedimentary rocks like limestone or sandstone – and has strong seasonal rainfall followed by long hot and dry periods.  During heavy rain  the water carries the silica from the rocks creating a silica-rich solution which finds its way deep into the earth, below the water table.  During the dry months the sun dries out the earth causing the water table to evaporate and retreat, leaving microscopic spheres of silica behind.  This process, repeated over millions of years, slowly creates layers of the silica spheres, and this is what forms opal.

Not all the opal formed will show the spectral hues we all love. A lot of what forms is called common opal, which can be a non-descript bluish-black or milky white colour.  The opal that shows the brilliant play-of-colour (or what many people refer to as ‘fire’) is known as precious opal.  Most opals are cut into gems with a component of common opal or host rock still attached, as it plays an important role in the look and strength of the gemstone.

The colour variation found in potch. Both of these specimens are black opal.  Image: Giulians
This is a large specimen of mostly common opal – with a small section of precious opal showing blue-green play of colour. Image: Giulians

Why is it found in Australia?

The reason opal is found in Australia is due to the Great Artesian Basin.  An artesian basin is an area where a body of groundwater is confined under the earth and held under pressure by surrounding layers of rock.    The Great Artesian Basin is the largest and deepest artesian basin in the world, covering 23% of Australia and stretching 1,700,000 square kilometres below arid and semi-arid parts of Queensland, New South Wales, South Australia and the Northern Territory.  It has a total volume of water of approximately 64,900 million megalitres.

The Great Artesian Basin was formed about 130 million years ago by a sheet of quartz over a shelf underground.  Rain ran off the mountains of the Great Dividing Range creating rivers.  The water on the eastern side ended up in the ocean while the water on the western side had no-where to go and just sat and sank into the earth. This process occurs every wet season and has been occurring ever since the Dividing Range was formed after heavy rains.  If you look at the opal fields in relation to the basin, it is little wonder why opal is mined in these locations.

Map of Australia, showing the Great Artesian Basin in light blue, in correlation the 3 major opal fields. Image: (TentotwoGreat Artesian Basin, added locations by ACoffey, CC BY-SA 3.0)

 

What causes the amazing colours seen in precious Australian Opal?

The rainbow colours seen in opal comes from the dispersion of white light through the layered spheres of silica.  For this to occur, however, the layering and stacking of the spheres of silica need to be both uniform in size and perfectly structured.  Without this structure, the result is common opal, or what we call ‘potch’ in Australia.

Scanning electron microscope imaging of the silica spheres of gem grade opal. Photo courtesy: www.adelaide.edu.au
Scanning electron microscope image of common opal. Notice the irregular and non-unified structure. Photo courtesy: Renac Christophe

The stacking and uniformity of the silica spheres is not the only factor involved. The size of the silica spheres determines the range of colour seen in the precious opal.  Smaller clusters of stacked spheres will show blue-greens, while the larger spheres will show the rarer colour; red.  The conditions under which opal is formed generally wouldn’t allow for a lot of space, so the smaller spheres (and therefore the blue-green colours) are more commonly found.  Rare and bright reds, combined with dark coloured potch is said to be the ultimate in opal. It is referred to in the industry as ‘black on red’ and will be considered very valuable.

The colour seen in opal is directional.  Some opals can display a blue-green flash in one direction and orange- red in another, while others may have shifting or rolling colour flashes.  It all depends on the angle of the stone and the way the light is hitting it.  A stone that has bright colour in all directions will be valued higher than an opal with colour in only one direction.

Full rotation of a 2.67ct Australian black opal showing the way the colour patches shift and change as I move the gem. Image: Giulians

The best thing about the colours found in opal is the diversity.  No other gemstone has such a variety of colour, pattern and combinations, so personal taste plays a major role in which opal appeals to you!

A selection of Giulians black and boulder opal pieces showing the wonderful diversity of colours and patterns.  Image: Giulians

 

Black Opal – Lightning Ridge, New South Wales

The most valuable of Australia’s opal is Black opal.  Black opal is found in sandstone, mined approximately 20- 30 metres below the ground.

#1 Common Opal or Potch, #2 Precious Opal, #3 Sandstone. Image: Giulians

Because it forms in sandstone, a relatively soft rock, it allows for deeper deposits to form.  Some of these deposits are called nobbies.  A nobby is bulb shaped opal nodule found in clay which can be quite large or the size of a pea.  They are tumbled to remove the sandstone, then ‘rubbed’ to allow the miner to look at the colour and quality.  Nobbies are mostly mined in Lightning Ridge – which is why black opals are known for being cut into lovely high-domed cabochons.  Other deposits of opal, like seam opal, occur in veins and bands and sometimes yield thinner and more irregular shaped gems.

A nobby of black opal – which has been tumbled to remove the sandstone and rubbed to reveal the opal. Image: Giulians

I thought black opals were supposed to be… black!

Many people who come to view our black opals often make this observation.   The truth is quite the opposite!  The potch found in Lightning Ridge has a darker body colour than that of white opal – and that is where black opal got its name.  The darker the body colour, the better the contrast between the common and precious opal – resulting in more vivid and striking play-of-colour in the gemstone.  The black opal potch can vary from inky-black through to very light grey – the light grey tones resulting in softer colours.  The potch is almost always kept as part of the finished gemstone – to provide thickness, strength and colour depth.  In thicker colour bands, black opal can be cut without its potch – and this is sometimes referred to as black crystal or jelly opal.

A 4.85ct black opal with almost no common opal or potch left behind it. Image: Giulians
Full rotation of a 9.39ct black opal showing blue-green play-of-colour. Image: Giulians
The natural potch left on the base of a black precious opal. Image: Giulians

Boulder Opal – Quilpie, Queensland

First discovered on a station south of Quilpie in 1872, boulder opals are found in a belt stretching from Quilpie to Winton in Outback Queensland.  Natural boulder opal is very unique.  Unlike black opal, it is formed in an area where the geology is made up of ironstone.  Ironstone is a dense sedimentary rock that formed in the Cretaceous period.  Due to this density, the host rock is very unyielding, so the opal formed in cracks and fissures within the ironstone.  These are referred to as veins, and they can vary in thickness from fine through to thick.

An example of the dense ironstone, with veins of opal running through it. Image: Giulians

Because the majority of boulder opal veins are thin, the ironstone boulder is cut as part of the gemstone – giving the opal its name.  Miners split the boulders through the veins, leaving the thin opal on the surface with the ironstone behind.

A vein of opal through ironstone – and how it has been ‘split’ open. Image: Giulians

Due to the reddish brown to brown-black colour of the ironstone, the play-of-colour seen in boulder opal can be very similar to that of black opal.  The natural ironstone making up some of the carat weight also means they are generally cost a little less.  You will also get more free-form and undulating shapes in boulder opal, as the miners have to follow the natural directions of the opal vein.

A stunning piece of Australian Boulder opal showing an undulating surface where the miner has had to follow the natural vein. Image: Giulians
This piece of boulder has a beautifully polished ironstone base. Image Giulians

White Opal – Coober Pedy, South Australia

The name Coober Pedy is said to have evolved from a combination of two Aboriginal words meaning ”white man in hole”.  Kupaka – meaning white man and Piti – meaning hole.  White opal forms in limestone which, like sandstone, is a soft host rock allowing deeper seam deposits to form. The body tone of white opal is much paler, so the play-of-colour seen is more bright pastel hues.  It is not uncommon for white opal to be cut without its potch which means the opal play-of-colour will be seen from the front and the back.

Limestone with some white opal potch. Image Giulians
A rough piece of white opal – displaying a milky body colour with a little pastel fire. Image: Giulians
White crystal opal with very bright play-of-colour and with very little potch on the back of the gemstone. Image: Giulians

I hope you have enjoyed learning about the geology and origins of Australian opal in Part 1 of our Opal Discovery!  In part 2 I will be looking more in depth at the colours and patterns found in opal, and the differences between them.  If you have any questions about opal or would like to know more about our opals, please get in touch by using the form below or emailing us giulians@giulians.com.au.

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Sources and Links:
http://mininglink.com.au/site/lightning-ridge
http://www.opalsinformation.com/index.php/articles/terminology/130-opal-terminology
https://www https://www.cooberpedy.sa.gov.au/page.aspx?u=368#.W7wjpXszapo.ritas-outback-guide.com/boulder-opal.html
http://www.expedition360.com/australia_lessons_science/2001/08/the_great_artesian_basin_forma.html

 


By Alice | Posted on October 9, 2018

We went on a road trip!

Last month, Gary, Trina, Joel and I took a day off work, and drove down to Canberra, to see Cartier’s exhibition of historical jewels on display at the National Gallery of Australia.  Established in 1847 in Paris, by Louis-François Cartier, Cartier is today an internationally recognised jewellery and watch manufacturer.

A beautiful day for a drive.

Famous for their garland style designs from the early 1900’s, it is of little wonder Cartier became known as the ‘Jeweller to Kings’ –  creating tiaras and ornate adornments for aristocrats, socialites and royalty.

Cartier Paris Tiara – 1905, choker necklace & Lily stomacher brooch 1906. Platinum and diamonds.

Cartier was heralded as a pioneer in the Art Deco style – their early use of minimalist and geometric shapes in their jewellery predated the First World War, and continued well into the 1930s.

Cartier Paris Brooch – Rock crystal, diamond, enamel, mother of pearl and sapphire -1924

During the late 1940’s the panther motif was introduced to their collection and became a quintessential Cartier symbol across their entire range, known as the Panthère de Cartier.  From pave sections adorning handbag clasps – mimicking the spots of a panther, to the three-dimensional panther brooch owned by Wallis Simpson (The Duchess of Windsor), the Panthère de Cartier collection has had many famous advocates – and is still going strong today!

Cartier Paris Brooch Watch – the reverse side of the rectangular Panther motif section has a watch dial – 1928

Wandering through the exhibition, it was interesting to see how the motifs evolved over the decades, and to see where the designers drew their inspiration from.  The archaeological exploration of Egypt, travel of the Royal family to India, and the art of East and Southeast Asia opened up an exotic source of inspiration, which they applied to all sorts of creations – cigarette cases, jewellery, clocks and watches.

Cartier Paris Bangle – Coral, emerald and onyx – 1930

Over 300 pieces were on display – a combination of Cartier’s collection and loaned pieces from private collections worldwide.  The jewels of Grace Kelly (Later Princess Grace of Monaco), Elizabeth Taylor, Sir Elton John and the Royal family were part of the exhibition.

Cartier Halo tiara made in 1936, belongs to Queen Elizabeth II and was worn by Kate Middleton on her wedding day in 2011

Before heading back to Sydney, Joel and I stopped at Lerida Estate Winery opposite Lake George, for a late lunch and a glass of their pinot rose.  When a winemaker compares the colour of their wine to an Argyle pink diamond, you know they have taste!

Scroll down to see a few more images taken on our journey.

The entry into the National Art Gallery of Australia, Canberra, ACT
‘The Garland Style’ – Cartier Paris Devant de Corsage – Platinum, diamond and pearls – 1902
Guilloché enamel barometer – Silver, gold, silver gilt enamel – 1908
Watch necklace – 1910. Guilloché enamel is a technique involving engine turning a pattern on gold, then firing with a vitreous transparent enamel over the top.
Cartier Paris bracelet watch – c.1910
Cartier Paris Snake Necklace – Platinum and diamond – 1919
Cartier Vanity Case – c.1920
Cartier London sautoir – Diamonds and rubies – 1924. This piece was amazingly flat in profile.
Cartier Paris for Cartier New York Panther pattern evening bag – 1924
Trina and Alice looking through one of the double-sided displays. The craftsmanship is not limited to the front of pieces, with the other side showing as much detail as the front.
Cartier Paris Vanity case – this is actually a photo of the back – 1924
Cartier Paris vanity case – 1925
Cartier powder compact and lipstick holder – 1925
Cartier Paris necklace – special order for the Maharaja of Patiala – 1928.
Two coral bracelets – inspiration from the East – 1930
Cartier London Collier – Platinum, Emeralds and diamonds – 1932
Cartier Paris – Imperial Jadeite strand – 1934. Like Australian opal, magnificent jadeite commands some of the highest prices among gems in today’s international market.
Cartier Bib Necklace – Gold, Turquoise and diamonds – 1955
Cartier Bangle – Gold turquoise and diamond – 1953
Diamond setters are a different part of the jewellery trade. Jewellers create the mountings, then the setters place and secure the gemstones.
Many of the hand tools we use today are the same as the ones used by jewellers in the 1800s. Hammers, files, saw-frames, blocks and punches have not changed over the centuries.
The official book of the exhibition, with stunning close up photographs of the pieces viewed.
Lerida Estate, Lake George – the perfect spot for a late lunch.
Inside Lerida Cafe – with the winery next door.
Lerida Estate Pinot Rose – a good drop!
Taking in the lovely views of Lake George from the winery, before heading home.

 

 

 

 


By Alice | Posted on August 28, 2018