What Is Onyx Gemstone?


Curious about the onyx gemstone? Here’s everything you need to know about onyx, its properties, and how to recognise a real black onyx gemstone from the rest.

What is onyx stone?

Onyx is a variety of stone that comes from Chalcedony. Chalcedony is a silicate mineral, mostly made of calcite (similar to marble and granite) and it usually features contrasting layers of colour.

What does onyx look like?

Most people associate onyx gemstones with a jet black colour with some contrasting white layered bands. However, onyx is also found in brown, red and white variations. Brown and white onyx is usually referred to as sardonyx, while the red and white variation is recognised as cornelian onyx.

All forms of onyx gemstone feature multiple layers, with the lighter layer of stone sitting on top of the darker layer. This property has, historically, made onyx a popular stone to use for engraving techniques as well as producing cameos, where a raised relief is produced to form a design that is lighter than the layer it sits on.

What is onyx made of?

Onyx is naturally formed from calcite in caves around the world when water drips and evaporates from the ends of stalagmites and stalactites. Once it evaporates, it leaves behind traces of minerals and calcium carbonate which causes its intricate bands of varying colours. The bands of colour are produced with alternating silica minerals – quartz and morganite – and leave behind bands that lie parallel to one another. This tends to be why agate is often mistaken for onyx, as it is formed in the same way. However, the layers produced when agate forms are not parallel. Instead, the alternating bands are produced in a more dramatically curved formation.

Where does onyx come from?

Onyx is mined from several countries around the world including:

  • Australia
  • Argentina
  • Brazil
  • Botswana
  • Madagascar
  • Mexico
  • Myanmar
  • Pakistan
  • Yemen

Onyx is also mined in some areas in the US.

What are the properties of onyx?

The properties of onyx include: 

Chemical Formula: SiO2
Mohs Hardness: 6.5-7
Density: 2.58-2.65
Refractive Index: 1.530-1.540
Luster: Vitreous

What is black onyx?

Black onyx is often the most well-known variation of the onyx gemstone. It’s a form of onyx that has a black base and white bands that sit parallel across the top surface of the stone.

onyx gemstone

What does raw black onyx look like?

Raw black onyx is usually not completely black. It can appear brown/greyish and sometimes dark green in colour when found in its natural state. But it appears black because it’s so deep in colour and opaque. That’s why a lot of jewelers tend to use a form of chalcedony that has been dyed black to get that deep, jet black that consumers are looking for, instead of slight hues of green and brown.

Raw black onyx is incredibly difficult to come by as it is a lot easier for suppliers to mine red and brown onyx and then dye it black. So be careful when it comes to buying onyx. If buying real black onyx is a must for you, you’ll need to have the knowledge to differentiate between what’s real and what has been dyed black. If you don’t have the experience to make out the difference between a real black onyx stone and one that has been treated, you may want to get in touch with an experienced gemologist to identify the stones for you.

In the industry many jewelers will use a dyed black agate stone or other common black stones such as black spinel, or jade and call it onyx. So, make sure that you trust your supplier’s knowledge, especially if using ethically sourced materials is a large part of how you work.

Is black onyx valuable?

Due to its rarity, black onyx can be expensive. If it’s identified by a gemologist as real, naturally occurring black onyx then it can be incredibly valuable. However, dyed onyx is very affordable to work with and can add a unique look to your designs without having to source more valuable stones such as black diamonds.

What is black onyx good for?

In the jewellery industry, black onyx is often used to create smooth cabochon stones for use in rings and pendants. Due to its smooth, glossy lustre, it’s a great stone to set with silver or gold pieces, and as it’s reasonably inexpensive to stock up on it makes for a popular choice among jewellers and consumers.

Where can I buy onyx stones?

We have a range of pre-cut onyx gemstones at Cooksongold. Check out the varied sizes and shapes available in opaque black and with varying banding detail. With our pre-cut onyx stones, it’s simple to add them to your pendant, ring or bracelet designs. Want to add an intricate design to your setting too? Take a look at our range of gallery strip for a different take on your latest stone set designs.



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