Onyx: Properties, Facts and Photos

What is Onyx?

Onyx is not a mineral in its own right, but a banded variety of chalcedony, which is a microcrystalline form of quartz. Microcrystalline means its crystals are too small to be seen without magnification. 

It's used for a variety of purposes, including gemstones, fireplace surrounds, household furniture, and ornaments.

There's a great deal of confusion surrounding the correct use of the name 'onyx,' and in most cases, it's used incorrectly.

The name has been applied broadly, meaning some materials may not adhere to strict mineralogical definitions.

Regarding the colour of onyx, it's a black and white, banded variety of agate, but can also have parallel bands in very similar shades of colour.

Although onyx and agate are both varieties of the mineral chalcedony, the main difference is onyx has parallel bands, which are typically black and white but can also include other colours. Agate has curved or concentric bands, which can vary widely in colour and size.

What is Black Onyx?

Most black onyx being sold is either black agate or black chalcedony. Natural black onyx is quite rare.

Throughout history, the vast majority of black onyx has been dyed. In one respected gemstone reference, it's stated that in order to achieve the black colour, stones are soaked in sugar or honey before being treated with sulphuric acid.

In his encyclopedia, Naturalis Historia [circa 77-79 AD], ancient Roman author and naturalist Pliny the Elder speaks at length about techniques used to change the colour of one gemstone to make it look like another.

"There is considerable difficulty in distinguishing genuine stones from false; the more so, as there has been discovered a method of transforming genuine stones of one kind into false stones of another.

Sardonyx, for example, is imitated by cementing together three other precious stones, in such a way that no skill can detect the fraud; a black stone being used for the purpose, a white stone, and one of a vermilion colour, each of them, in its own way, a stone of high repute.

Nay, even more than this, there are books in existence, the authors of which I forbear to name, which give instructions how to stain crystal in such a way as to imitate smaragdus and other transparent stones, how to make sardonyx of sarda, and other gems in a similar manner.

Indeed, there is no kind of fraud practised, by which larger profits are made."

Ref; (Pliny's Naturalis Historia chapter 75). Please Note.

Onyx or Calcite?

It can sometimes be difficult to tell onyx from calcite on sight alone. The simplest way to identify one from the other is to do a scratch or acid test, but the problem is that both will cause some amount of damage.

On the Mohs scale of hardness, onyx grades 6.5 to 7. Calcite grades 3, making it far easier to scratch.

To establish whether the material is calcite, you could try scratching it with fluorite or a feldspar mineral such as labradorite. Alternatively, you could use window glass, which is harder than calcite but softer than onyx.

Quartz, which also grades 7, may scratch it, corundum, topaz, and diamond definitely will. 

An acid test, using something like vinegar, will determine whether the stone is calcite or onyx.

Calcite is the crystalline form of calcium carbonate, so when it comes into contact with acid, it effervesces (fizzes). This happens because of a chemical reaction between the acid and the carbonate ions in calcite. 

When acid comes into contact with calcite or another calcium carbonate mineral, tiny bubbles appear. They can be very small, so may be difficult to see. If the material is onyx, it will not effervesce.

Stones with light-coloured, alternating parallel bands, often sold as onyx, tend to be calcite, chalcedony, agate, or any number of other minerals.

Some History of Onyx

Onyx is mentioned numerous times in the Bible and is said to have been one of the gemstones in the high priest breastplate.

This biblical garment was worn by Aaron, the first Jewish high priest, over a tunic-like garment.

Onyx is said to have been the second stone in the fourth row. To its left was a beryl, and to its right a jasper.

Throughout history, the name onyx has been applied to a wide range of materials, contributing to the confusion that still surrounds it today.
Much of the 'onyx' used for decorative purposes, such as chess sets, figurines, tumbled stones, and gemstones, is actually banded calcite. The stone in the image above is a good example.

Calcite is often used instead of onyx because it's more readily available and much softer, making it easier to work with.

Onyx: So Much Confusion

The name onyx describes a banded variety of agate. Agate is a type of chalcedony, which in turn is a type of quartz.

Terminology can be ambiguous, and the translation of names into different languages, especially over hundreds or thousands of years, has further complicated matters.

In historical texts, lapis lazuli was often mistaken for sapphire because, in the ancient world, it was known as sappheiros.

Determining the exact characteristics of some minerals can be subjective and open to interpretation. Geologists often have different criteria and perspectives when defining onyx, which can lead to disagreements.

Using trade names to popularise rocks and minerals also causes confusion. Many stones that have been given a name that ends with 'Jasper' do not contain any jasper at all. An example is Dalmatian Jasper. 

Limestone onyx is a trade name for limestone that resembles onyx in appearance due to its banding, but rarely has the same properties.

'Green onyx' is often agate or calcite that's been dyed.

'Onyx' or 'onyx marble' from Pakistan is travertine composed of colourful bands of calcite.

The stone in this photo, which is calcareous onyx, is a rock, not a mineral , because it's an aggregate of minerals, mostly calcite or aragonite.
Calcareous onyx forms from calcium carbonate deposits, typically in caves or around hot springs. Its banding, which can be similar to true onyx, is created by layers of calcite or aragonite rather than silica.

Article Pictures

I'm grateful to several photographers for allowing me to use their work on our website. However, it was extremely difficult to find photos of natural onyx.

Most material labelled as 'onyx' is calcite or contains a significant percentage of calcite.

The photo at the top of our article, courtesy of Stan Celestian, is natural onyx.

Pop-up photos: Rootbeer onyx: Courtesy of Stan Celestian. The Dalmatian Jasper tumbled stones are from our collection. Corundum and lapis lazuli: Courtesy of James St. John.  

I've added links to these next two photos of onyx because I was unable to contact the photographers to request permission to use them on our website:

https://www.mindat.org/photo-49959.html
https://www.mindat.org/photo-718344.html 

The remaining photos in our article are of banded calcite. The rectangular gemstone is from our collection.

The calcareous calcite is from a social media post. Photo: Bruce Francis Taylor.