Green Onyx Vs. Banded Calcite
If you’ve ever browsed crystal shops or online marketplaces, you may have come across two green stones with the same name: green onyx. At first glance, these stones look completely different but why do they share the same name? Well, that’s because neither of these stones are green onyx! Let’s break it down so you’ll never confuse the two again.
What Is Onyx?
Onyx is a variety of chalcedony, which itself is a form of polycrystalline quartz. Traditionally, the term ‘onyx’ refers to a material with parallel banding, typically in black and white. While black onyx is well-known, other natural colours like brown or white can occur. Today, a lot plain black chalcedony is referred to as ‘onyx’. To top it off, a lot of the plain black onyx material on the market is treated with sugar and sulphuric acid. Onyx forms in silica-rich environments, often in volcanic rocks or hydrothermal veins. Its parallel banding occurs due to the sequential deposition of silica layers.
Green Onyx That Is Actually Dyed Chalcedony
‘Green onyx’ is not a naturally occurring form of onyx. Typically in jewellery, it has been used to refer to chalcedony that has been dyed to achieve its vibrant green colour. Chalcedony is a microcrystalline quartz, and while it can occur naturally in various shades, bright green is rarely among them. As a result, the green tones seen in ‘green onyx’ are typically the result of artificial colouring processes. There are however, a number of other varieties of green quartz which I’ve listed further down in this article.
Understanding the distinction between natural and dyed materials is essential when evaluating the true value of ‘green onyx’. The term used for dyed chalcedony is confusing buyers and inflating prices. Understanding this trade practice is key to making informed purchases.
Green Onyx That Is Actually Banded Calcite
Banded calcite on the other hand is a carbonate mineral and, despite its jade-like nickname, has no connection to true jade (nephrite or jadeite). It’s characterised by its distinctive banding, with layers of green, white, and occasionally brown or yellow. The term ‘Afghan jade’ is a marketing name that capitalises on its resemblance to jade, but it’s important to note that this stone is much softer and chemically distinct.
Banded calcite forms in sedimentary environments, typically as a secondary mineral in limestone caves. It develops from the precipitation of calcium carbonate from water, creating its signature bands through variations in mineral content and flow conditions.
Because of its softness, banded calcite is best suited for ornamental pieces like carvings and display stones rather than wear-and-tear jewellery. It’s also worth noting that many sellers of kitchen counter tops incorrectly label this material as ‘onyx’.
How to Differentiate Between Onyx and Banded Calcite
Banding and Texture
Onyx typically has an even texture, with banding that’s parallel and consistent.
Banded calcite often displays layered patterns of green, white, and other hues, giving it a marbled appearance.
Transparency
Onyx is often slightly translucent but more uniform in appearance.
Banded calcite has a waxy or silky lustre and a more varied translucency due to its banding.
Weight and Density
Onyx feels heavier due to its higher density, while banded calcite is relatively lightweight.
Hardness Test
Onyx, being quartz, can scratch glass or a steel blade. Banded calcite, on the other hand, is so soft that even a copper coin can scratch it.
Reaction to Acid
Banded calcite will react to dilute acids like vinegar, fizzing as it dissolves. Onyx (quartz) will not react to acids.
Why the Confusion?
The confusion often arises because of marketing tactics. Vendors may label banded calcite as ‘Afghan jade’ to command higher prices, capitalising on jade’s popularity. Similarly, dyed chalcedony is often sold as ‘green onyx’ without clarification, further muddying the waters. Knowing these differences empowers buyers to make informed choices.
Other Varieties of Green Quartz
In addition to dyed chalcedony marketed as ‘green onyx’, several natural varieties of green quartz are worth exploring. These varieties differ in their origins, chemistry, and visual properties:
Prasiolite
This is a rare, naturally occurring green quartz that gets its colour from iron impurities. Most prasiolite on the market is produced through the heat treatment of amethyst or citrine. These are also typically sold as ‘mint amethyst’ or ‘green amethyst’.
Aventurine
A type of quartz characterised by shimmering inclusions of mica or other minerals, which create a sparkling effect. Aventurine often comes in shades of green and is prized for its textured appearance.
Chrysoprase
A green chalcedony coloured by nickel content. Unlike ‘green onyx’, chrysoprase’s vivid green colour is entirely natural and more opaque, often appearing milky.
Prase
An opaque variety of green quartz with a softer, muted green colour. It owes its hue to inclusions of actinolite or chlorite. Prase is less commonly seen in commercial markets but is appreciated by collectors for its natural and earthy tone.
These natural varieties are distinct from ‘green onyx’ in terms of origin, formation, and authenticity. Being familiar with their characteristics will help you identify what you’re purchasing and avoid confusion in the market.
A Word of Advice
Reputable sellers will disclose whether a stone has been treated or not and will avoid using misleading names like ‘Afghan jade’ or ‘green onyx’. Trustworthy vendors are transparent about their products but sadly there are many wholesalers and very popular jewellery brands that are not disclosing this on their websites. If a store is selling ‘green onyx’ but not disclosing it has been dyed, then buyer beware.
Further Reading & References