Onyx is a banded variety of the oxide mineral chalcedony. It ranges in color from black to white to almost every color in between. However, onyx most commonly refers to the gemstone that is black and white banded. So is onyx black or grey? The answer is that onyx can be both black and grey depending on the specimen.
The Origin and Formation of Onyx
Onyx forms when silica from lava or hot spring water is deposited inside the gas vesicles of certain rocks over time. As layer upon layer of silica coats the walls of the vesicles, alternating bands of different colors are created based on the trace impurities present. The signature black and white banding of onyx is formed when silica is deposited around layers of black carbonaceous material in the host rock.
Onyx deposits are found in Mexico, the United States, Peru, India, Madagascar, and Brazil. The colors and patterns of onyx from each locality can vary based on the impurities present during formation.
The Color Variations of Onyx
While black and white banded onyx is the most iconic, onyx can exhibit a wide range of colors. Here are some of the common color variations of onyx:
- Black onyx – Solid black or black with white banding
- Grey onyx – Solid medium grey or grey with white/black banding
- White onyx – Solid white or white with grey/black banding
- Red onyx – Bands of red, white and black
- Green onyx – Bands of green, white and black
- Blue onyx – Bands of blue, grey and white
- Yellow onyx – Bands of yellow, white and grey
The color of onyx depends on the trace minerals present when it formed. For example, iron oxides produce red, yellow and brown. Copper results in green and blue. Manganese creates pink shades. Carbon creates black, while pure silica is white.
Properties of Black Onyx
Black onyx refers to the black bands or solid black sections of the onyx stone. Here are some key properties of black onyx:
- Color – Jet black, sometimes with white banding
- Luster – Vitreous to waxy
- Transparency – Opaque
- Streak – White
- Mohs hardness – 6.5 to 7
- Refractive index – 1.544 to 1.553
- Specific gravity – 2.58 to 2.64
The black color of onyx is caused by carbonaceous material inclusions. The hardness, streak and vitreous luster help distinguish black onyx from black obsidian, which has a glassy luster and density.
Properties of Grey Onyx
Grey onyx exhibits lighter grey to charcoal grey shades. Here are the properties of grey onyx:
- Color – Greyish black to medium grey
- Luster – Waxy to dull
- Transparency – Opaque
- Streak – White
- Mohs hardness – 6.5 to 7
- Refractive index – 1.544 to 1.553
- Specific gravity – 2.58 to 2.64
The grey shades of onyx are caused by lower carbon content during formation. Grey onyx lacks the vivid black bands but often has subtle banding. It shares the same physical properties as black onyx.
Distinguishing Black and Grey Onyx
While black and grey onyx have overlapping properties, there are some visible differences:
| Black Onyx | Grey Onyx |
|---|---|
| Deep black color | Lighter charcoal grey |
| Highly visible white banding | Subtle or no banding |
| Very opaque | Lower opacity |
| Higher carbon content | Lower carbon content |
The most obvious difference is the intensity of the black color. Under light, grey onyx usually appears translucent near the edges while black onyx remains fully opaque. The banding patterns are also more vivid in black onyx. Chemical testing would identify a higher carbon content in black onyx.
Uses of Black and Grey Onyx
Both black and grey onyx are used for similar purposes as gemstones and decorative stone. Some of their common uses include:
- Cabochons for jewelry making
- Beads for necklaces, rosaries and bracelets
- Intaglio carvings and cameos
- Sculptures and figurines
- Interior decoration – floors, walls, furniture
- Vases, bowls and other ornamental objects
The black and grey banding patterns of onyx make it ideal for carving. Solid black onyx is often used for mourning jewelry, while banded black and white onyx is popular for statement jewelry. Grey onyx is softer and subtler, lending itself to minimalist designs. Both varieties add elegant style to home decor.
Value Differences
In general, black onyx is more valuable and desirable than grey onyx in the gem trade. Here are some of the reasons for the value differences:
- Color intensity – The rich black color has stronger visual appeal.
- Banding – The distinctive banding increases value and uniqueness.
- Opacity – Black onyx’s full opacity allows better carving definition.
- Mystique – Black onyx is linked with various metaphysical and cultural meanings.
However, supply also plays a role in determining value. Grey onyx is more abundant since black onyx forms under specific conditions. From a rarity standpoint, jet black onyx with high contrast banding can reach higher values than common grey onyx.
Conclusion
In summary, onyx comes in a range of black and grey shades depending on its composition. While they overlap in properties, black onyx exhibits a deeper black color and more visible banding patterns. The vivid black of quality onyx is highly valued in jewelry, while abundant grey onyx serves well for carvings and ornamental objects. Both black and grey onyx make striking additions to any wardrobe or home decor.