Opal

This is the parent section for our Opals – we sell a wide range of stones including Welo Opal, Australian Opal, Honduran Opal and more.

Opal is a gorgeous gemstone which often shows a play of colour with many bright colours available.

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Appearance, Uses and History

Opals are probably my favourite ‘stones’. Looking into a good quality Opal is like staring through a telescope into a distant nebula, or a galaxy lighting up the sky.

They are technically a mineraloid, not a mineral, as they are amorphous and does not possess any crystalline structure.

In the most basic terms, there are two types of opal – precious and common. Precious opal has play of colour, an iridescence with bright colours – common opal does not have play of colour. However, I do go into more depth on this below..

Opal was extremely rare in history, with a single village in Slovakia producing all the Opal known to Europe, where it was worn by Roman senators, emperors, British royalty and other members of the ultra rich.

It was not until the discovery of large deposits in Australia that Opal became more available – since then, further deposits have been found and the price of opal has become much more achievable for most.

It’s difficult to explain what an opal looks like.

Common opal can simply be a white, pink, green, blue, black blob, not exactly anything to write home about.

Hyalite Opal looks like a brain made of glass, and shines brightly under UV light.

White Opals with good play of colour are pretty, with flashes of colour showing greens, blues, reds, purples – however, they do not have the amazing contrast that precious black Opal possesses, which makes the colours pop out so much more.

Locales

Australia

Writing an article on Australias opal fields would be an entire post in itself – probably several posts. I will quickly cover the most famous and well known fields and types of Australian Opal though.

Australian Opal is mined in three states; Queensland, New South Wales, and South Australia.

In Queensland, the main fields are Koroit, Opalton, Quilpie, Winton, Yowah and a few smaller fields are collectively known as the ‘Queensland fields’ which primarily produce boulder opal in ironstone.

In New South Wales, the main fields are Lightning Ridge, the Grawin, and White Cliffs.

  • Lightning Ridge is one of the best known opal fields, providing much of the best black opal in the world.
  • The Grawin refers to a group of smaller opal fields near Lightning Ridge; it is a very unique outback village. There’s also a golf course, if thats your thing!
  • White Cliffs was Australias first commercially mined opal field, producing white opals and many pieces of fossil opal.

 

In South Australia, the main fields are Andamooka, Coober Pedy, Lambina, Mintabie.

  • Andamooka is known for a matrix opal which is often sugar treated to improve its colour.
  • Coober Pedy is probably one of the most well known opal fields; maybe because of its unusual name, quality material, and unusual underground structures – due to the high heat of the area.
  • Lambina is a relatively new opal field with decent white to black opal.
  • Mintabie is now a closed field as the state government did not renew its lease.  Previously, the field produced white-grey backed opal with bright play of colour and rarely, some grey-black and crystal opal.

 

Ethiopia

Both volcanic and sedimentary Opal can be found in Ethiopia – the volcanic opal from Shewa province tends to occur in nodules with rhyolite, and mostly has a dirty/dark brown or dirty orange backing colour; it also tends to crack if cut.

The opal from the Wollo province, sold as Welo or Wello opal, often has a clear, white, or light orange, light brown backing colour and some quite bright flash. It is significantly cheaper than Australian Opal on average, but most of the Wollo province opal is hydrophane, and will absorb water readily. This leads it open to cracking or losing its play of colour; obviously, not something you want in a piece of sold jewellery!

Ethiopian Opal is often smoke treated or oiled.

 

Honduras

Honduran Opal is mined in the Lempira district, in the west of the country. The Opal from this area is found in volcanic black basalt, which gives the opal its dark base colour.

While there is precious Opal found in Honduras, the most well known is the Honduran matrix opal with its many small pinpricks of fire. It can be polished and used for jewellery, but ideally should be resin stabilised before this.

 

Indonesia

Indonesian Opal is found in the Banten province, of which much is a form of opalised petrified wood. There is also volcanic Opal. Most opal from Indonesia is treated, either with smoke, oil, or sugar treatments.

The Indonesian ‘wood opal’  is quite interesting; it typically consists of a dark black matrix or backing with bright play of colour. It is often in the shape of wood and may show wood grain etc – however, it is often carved into tree or branch like shapes.

 

Mexico

Mexico has a fair few Opal deposits in various states, including Queretaro, Chihuahua, Jalisco, San Luis Potosi, Guerrero – probably more!

There are several types of Opal found in Mexico.

‘Mexican fire Opal’ often has a bright yellow, orange, or red backing colour. It does sometimes have play of colour – pieces with colour are typically cut en cabochon; however, pieces without play of colour are commonly facet cut.

It often occurs in rhyolite or tuff, and when still in matrix, could be considered boulder opal. In Mexico, it is known as cantera.

Hyalite Opal occurs in Mexico, with deposits found in Zacatecas, San Luis Potosi, and Aguas Calientes.

Common Opal, crystal opal, jelly opal, and precious opal also occur in Mexico.

 

Peru

Peruvian Opal is found near the Andes mountains. It is primarily a common opal without play of colour, and has a strong blue or pink background colour. Unfortunately there are dyed imitations, becoming more prevelant. Genuine Peruvian Opal should have a fairly homogenous colour without paler sports.

 

USA

American Opal can be found in a few states, including Arizona, Arkansas, Idaho, Louisiana, Nevada, Oregon, Utah, and Washington. Nevada is the largest producer of Opal in the USA, providing both common opal and gem black opal.

Black opal, blue opal, common opal, fire opal, hyalite opal, pink opal, and opalised wood can be found in the US.

 

 

Other sources of Opal include sources in the Czech Republic, Canada, Slovakia, Hungary, Turkey, and, believe it or not, ON MARS. The existence of Opal on Mars helps to narrow down a timescale for when water would have existed upon Mars.

 

Mineralogy

Chemistry
Hydrated silica, SiO2•nH2O
Colours and Variations

Most colours are possible.
Types of opal include

  • Black Opal – an opal with a dark black body tone, and contrasting play of colour. Black always refers to body tone, rather than the play of colour itself.
  • Boulder Opal – opal formed in the seams, cavities, and cracks in a boulder of ironstone. Usually left in matrix and polished as a piece with both ironstone and opal.
  • Common Opal – a term typically used to describe opals without play of colour.
  • Crystal Opal – a very clear form of opal with a clear body tone and play of colour.
  • Dendritic Opal – common opal with inclusions of dendrites.
  • Fire Opal – primarily used to refer to Mexican Fire Opals, but sometimes used as a term for ‘play of colour’.
  • Fossil Opal – Opal of a fossil origin, which may not have obvious fossil shape if it is being sold as pre-formed for cutting.
  • Hyalite Opal – an unusual fluorescent form of Opal coloured by Uranium inclusions. It is also correctly known as Opal-AN.
  • Hydrophane Opal – hydrophane opal is porous enough to absorb water, which can lead to cracking or loss of colour.
  • Jelly Opal – similar to crystal opal, but a little bit milky – not perfectly clear.
  • Matrix Opal – opal in a matrix of some sort, such as the pinfire/pinprick pieces from Honduras, or boulder opals from Australia, Ethiopia, or Mexico.
  • Opalised fossils – fossils which have been replaced with Opal rather than silica or calcite. Recognisable as fossils to an extent.
  • Opalite / opaline – formerly used as a term for low grade/low quality opal. It should not be used anymore to avoid confusion with Opalite glass, a manmade glass.
  • Precious Opal – a catch all term for an opal with play of colour.
  • White Opal – opal with a pale to white body tone, which may or may not have play of colour. However, most people would refer to an opal without play of colour simply as ‘common opal’ or ‘potch’.

Other related and important terms include:

  • Aurora Opal – a form of synthetic Opal.
  • Body tone – body tone of an opal is used to determine its colour – rating from N1 as the darkest (black opal) body tone to N9 as the lightest (white opal). It helps when selling Opals online to distinguish the tone. N1-N4 opals are considered black, N5-N6 are considered dark, or sometimes ‘semi-black’, and N7-9 are considered white.
  • Doublet – a doublet is a manufactured stone, usually consisting of a slice of transparent opal glued over a black onyx or potch backing. In some cases, it may also be a quartz or glass topper over a slice of opal.
  • Fire – fire can either be used as a term to describe ‘play of colour’, for example “the opal has bright fire” or describe fire Opals, which have a deep orange-red body colour to them.
  • Gilson Opal – a form of synthetic Opal.
  • Nobby – a piece of lightning ridge opal with a natural lumpy shape.
  • Opalite – confusingly, Opalite formerly referred to a low grade of Opal, but now a man-made glass is on the market which uses the same name. To avoid confusion, I recommend Opalite is no longer used in reference to any genuine Opal.
  • Play of colour – an optical effect which causes flashes of colour when an opal is viewed under a light.
  • Potch – common opal, typically with no play of colour.
  • Ratter – a claim jumper, who steals from someone elses claim or mine.
  • Rough – an uncut opal which has not been cut or polished.
  • Seam – a seam refers to a horizontal crack or void which has been filled with opal.
  • Synthetic Opal – a catch all term for man-made artificial Opals.
  • Triplet – similar to a doublet, but three layers – typically a domed piece of clear Quartz or glass, then a thin slice of Opal, then a black backing, typically onyx or black potch.
Streak
White
Luster
Subvitreous to waxy, vitreous when polished
Fracture
Conchoidal
Transparency
Opaque to transparent
Crystal habit
Amorphous.
Mohs hardness
5 to 6.5
Specific Gravity
2-3, generally 2.15
Fluorescence
Sometimes, but this is very variable. Hyalite Opal is extremely fluorescent, others are not.
Easiest testing method
Common Treatments
Sometimes treated with heated sugar, oils or acids. Sometimes smoked – this is especially prevelant in Ethiopia and Indonesia.

Photos of Opal

This page would be huge if I included photos of each variation of Opal, unfortunately. I’d also probably lose six hours looking at photos of Opals.
Instead, I’m just going to post links to two excellent photo galleries:
https://www.gemdat.org/gem-3004.html
https://www.mindat.org/gm/3004

Hazards and Warnings

Almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis.

When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.

Translations

Arabic:

  • أوبال

Hindi:

  • दूधिया पत्थर

Portuguese:

Bengali:

  • ওপাল

Indonesian:

Punjabi:

English:

  • opal

Italian:

  • opale

Russian:

  • опал

French:

  • opale

Japanese:

  • オパール

Spanish:

  • ópalo

German:

Korean:

  • 오팔

Thai:

Gujurati:

  • ઓપલ

Mandarin and Traditional Chinese:

  • 蛋白石

Urdu:

Further Reading / External Links

  • https://en.wikipedia.org/wiki/Opal
  • https://geology.com/gemstones/opal/
  • https://www.gia.edu/UK-EN/opal
  • https://www.gemdat.org/gem-3004.html