What is Black Opal?

What is Black Opal? 

What is Black Opal is a question I’m often asked by friends living in Australia and my friends who live abroad in the United States, Canada, and the UK. So when my friends hear the word “black” they naturally assume that the black opal is black and often ask me this question “what makes a black rock so rare and incredibly spectacular”? Hmm, a good question, a question we will find out the answers to a little further down the page…

The Beginning

So let’s take a little look into 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 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 you currently own a solid Opal, jot 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.  WOW, just imagine how ancient your Opal could be.

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 percent by weight, but the ideal range is between 3 to 10 percent.
  • Quality Australian Opals contain around 5-6 percent 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.
what is black opal
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.

The Silica spheres are what orchestrates the amazing and unique light shows that captivate audiences from all over the World.

Australian black opal fire
Cmglee – Creative Commons

The size of 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 and can create the entire spectrum of colours.

Example 2. Refraction – when light travels from one medium that has a different density to another, 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 that displays every colour you can imagine. And this gemstone 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 hydrated Silica spheres 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 

  • The 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 percent
  • Secondary hue – 20 percent


  • The richness of colour


  • The depth and amount of fire


  • Display of colour when rotated


  • Large, vivid and clean adds value


  • Marks and inclusions lower the value

Carat Weight  

  •  One carat (1 ct) = 200 mg  

Shape & Proportion

  • Symmetry and shape can have an influence on gemstone price.


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

what is black opal

Thank you for stopping by and I hope you’ve enjoyed the information presented in this article.

For any questions, additional information or to make a comment, please do so in the comments box.

And finally, if you can let me know from which country you are from, that will be great.

Thanking you…

Related Links
Lightning Ridge black opal
Australian black opal pendants
Lightning Ridge opal mining

4 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.

  1. Your blog is great, grab so much intresting facts about black opal from your blog. I love all type of gemstone but black opal is something different which always puch me to know more about it. Thanks for sharing this.

    • Your most welcome and thank you for your kind comments. I love opal particularly black opal and Queensland boulder. If you need any further information that I can help you out with please do not hesitate to contact me. Cheers! Brad

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