What is Black Opal?

What is Black Opal? 

What is Black Opal is a question I am often asked by many people living outside of Australia. So before I begin my article about the Black Opal Stone, can I ask you if you are familiar with the Black Opal? If you could leave a comment and let me know if you have any knowledge about the Black Opal and from which country you are from, that would be fantastic. Thank you!!!

The Beginning

So let’s take a brief look at the beginning of Australia’s national gemstone the Opal. The formation of Opal began millions of years ago when an inland sea covered the central part of Australia. This sea provided our Earth with silica-rich deposits that found their way into cracks, voids, and decomposed fossils as the result of water flowing down through overlying sandstone. And still to this day we are finding marine fossils and opalised marine fossils in inland parts of Australia hundreds of miles away from any coastline.


Now if you happen to purchase an Opal through our website, or any other website for that matter, write down the measurements and remember this: Palaeontologists estimate that it takes approximately 5-6 million years for a 1 cm thick opal vein to mature. So, how ancient is your opal?

What is Black Opal? 

  • Unlike Diamonds, Opal is a non-crystalline, hydrated amorphous form of Silica, created by the combination of Silicon Dioxide (Silica & Oxygen) with water Si02.nH2O.
  • The moisture content of Opal varies between 3 to 21% by weight, but the ideal range is between 3 to 10%.
  • Quality Australian Opals contain around 5-6% water by weight.
  • Crystalline forms of silica are minerals. However, Opal is a mineraloid because it has no defined shape or crystalline structure (amorphous).
  • Opals have a relative hardness value of approximately 5.5 on the Mohs scale which is about the same as glass.
  • Opal, in my opinion, is a bit more fragile than glass so avoid contact with any hard surfaces.
black opal stone
Black Opal Stone

What is Black Opal? – Internal Structure

The internal structure of Black Opal is made up of microscopic spheres of Silica approximately 0.00005 – 0.0004 mm in diameter, that have grown around a central nucleus. Viewing these
minuscule areas of Silica requires the use of an electron microscope with 30,000x magnification.

Silica spheres are what orchestrate amazing light shows, and all shows are unique.

Australian black opal fire
Cmglee – Creative Commons

The size in the diameter of the hydrated Silica spheres determines the ‘play of colour’ by causing white light to refract and diffract.

Example 1. Diffraction – light waves bend as they pass the edge of an object and around small particles. Within Opal, the light turns around tiny spheres of hydrated Silica and oxygen. Light diffraction can create the entire spectrum of colours.

Example 2. Refraction – when light travels from one medium to another medium with a different density, light changes direction. Unlike a diamond, Opal doesn’t have a definitive Refractive Index (RI) because the refractive indices of the spheres of hydrated Silica vary. Therefore, the Refractive Index measurement for Opal can range between 1.37 – 1.52.

You can see in example 1, the longer wavelengths of red, diffract more than the shorter wavelengths of violet. In example 2, the shorter wavelengths of violet refract more than, the longer wavelengths of red.

Diffraction bends waves; refraction changes the direction and speed of the waves.

Brilliant and captivating colours display when light waves travel through the transparency of Silica, and it is possible for all the colours of the spectrum to be visible within just one stone. I know because I have a 2.86 carat solid Black Opal with a body tone N3, and a brightness 4/5. My Opal displays every colour you can imagine, and it just takes me to another world.

  • Most Opals consist of spheres of hydrated Silica less than 0.00025 mm in diameter. But precious Opals consist of larger silica spheres, which arrange in an orderly pattern creating spaces between the spheres.
  • An irregularly stacked, non-uniform in size opal, with no-show of colour, is called potch. This form of Opal is most often made up of silica spheres as tiny as 0.0001 mm in diameter. These spheres of hydrated Silica are way too small for light to diffract. Potch is a common Opal and the nucleus for precious opal to form.
  • Smaller-sized spheres produce violet colours.
  • Medium-sized spheres create blues and greens.
  • The rare colour of red is the result of larger spheres 0.0003 mm in diameter.

What is Black Opal? – Characteristics (C’s) 

A Diamond has four characteristics (4 C’s), however, Opal has many features including:

Opal type

  • Black 
  • White
  • Crystal
  • Boulder
  • Matrix
  • Pineapple

Crystal Opal – is the only form of Opal that does not contain potch and is transparent when put up to light

Body Tone 

  • Shade of the body
  • Darker tones display more vibrant colours


  • Faint:  1/5
  • Dim:  2
  • Somewhat Bright:  2.5/5
  • Bright:  3/5
  • Quite Bright:  3.5/5
  • Very bright:  4/5  
  • Extremely Bright:  4.5/5
  • Brilliant:  5/5


  • Opaque
  • Translucent
  • Transparent


  • Predominant hue 50 – 70 %
  • Secondary hue – 20 %


  • The richness of colour


  • The depth and amount of fire


  • Display of colour when rotated


  • Large, vivid and clean adds value


  • Marks and inclusions lowers the value

Carat Weight  – 1 carat = 200 mg  

Shape & Proportion 

What is Black Opal? – Home of the Black Opal Stone

A little place called Lightning Ridge in north-western NSW, Australia is home to the Australian Black Opal. Surrounding Opal fields of Grawin, Glengarry, and Sheepyards lie 60 – 70 km west of Lightning Ridge.

what is black opal
Aerial View of Lightning Ridge – M P Goodwin – Wikimedia Commons

Driving time to Lightning Ridge from Sydney is about nine hours. From Brisbane is just under eight and a half hours. The Gold Coast is approximately eight hours and 45 minutes. From Melbourne is around 13 hours. And from our nation’s capital, Canberra is eight hours and 30 minutes.

There are other modes of transport you can use to get to the Ridge. These include direct flights and train and bus transfers from Dubbo.

If you love gemstones, you will love the Australian Black Opal. And within our Opals n Jewels website, you will find an amazing array of solid Black Opal Stones all waiting for someone special like yourself to give them a home.

Fire Opal


Of all the different Opal types, the Black Opal is the most valuable because of its rarity and characteristics. Distinguished from other Opals by its darker body tone, the Black Opal actively enhances the refraction, diffraction, and reflection of light. And the most valued colour in Black Opal is the red fire which can fetch a price of up to Aud $20,000.00 for one carat.

what is black opal

Thank you for stopping by, and I hope you’ve enjoyed the information presented within this post.

For additional information or to comment, please do so in the comments box.

And finally, can you tell me your favourite colour.


2 thoughts on “What is Black Opal?

    • Hi Opal Direct,

      Thank you for your comment and I see you have some really nice opals on your website. They are a spectacular gemstone with some producing some of the most amazing light shows on earth, as you no doubt experience yourself. Since I began cutting opal I have developed a fascination with all the gemstones of the world. And my wife, who was one of the few women in the world who did not like jewellery, joined a lapidary club 6 days after she saw me cutting opal. She was hooked. And Opal has led me to discover, work with, and appreciate all gemstones.

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