Onyx is a popular gemstone that has been used in jewelry and ornamental objects for thousands of years. However, there is some confusion over whether onyx should be classified as a stone or a crystal. In this article, we will examine the properties and composition of onyx to determine if it is more accurately described as a stone or a crystal.
What is Onyx?
Onyx is a banded variety of chalcedony, which is a cryptocrystalline form of silica. Cryptocrystalline means that the silica crystals are so finely grained that they are invisible to the naked eye. Onyx has straight, parallel bands of alternating black and white colors. The black bands are composed of hornblende, while the white bands are composed of aragonite or calcite.
Onyx has a hardness of 6.5 to 7 on the Mohs hardness scale, which means it is softer than quartz but harder than glass. The luster of onyx can range from waxy to vitreous. Onyx has a conchoidal (curved) fracture pattern, which means that it breaks in a smooth, shell-like shape. The specific gravity of onyx is 2.6, which means it is denser than water.
Key Properties of Stones vs. Crystals
To determine whether onyx should be classified as a stone or a crystal, we need to compare its properties to the key defining qualities of stones and crystals.
Stones
Some key properties of stones include:
- Do not have a definite crystalline structure or regular geometric shape
- Composed of mineral grains cemented together in a solid mass
- Often have bands or layers
- Dull to waxy luster
- No clearly defined cleavage planes
- Irregular fracture pattern
Crystals
Some key properties of crystals include:
- Definite internal crystalline structure
- Regular geometric shape with flat faces
- May have banded growth lines
- Glassy, vitreous luster
- Cleavage planes related to crystalline structure
- Smooth cleavage surfaces
Is Onyx a Stone or Crystal Based on its Properties?
When we compare the properties of onyx to the defining qualities of stones and crystals, onyx matches better with the description of a stone rather than a crystal.
Here is an analysis of how onyx fits the stone criteria:
- Onyx does not have a regular crystalline structure, but rather is cryptocrystalline with micrograins
- It is composed of silica grains cemented together in a solid mass
- Onyx exhibits straight, parallel banding patterns
- It has a waxy to dull luster rather than a vitreous or glassy luster
- There are no clearly defined cleavage planes
- Onyx has a conchoidal (curved) fracture pattern rather than smooth cleavage surfaces
Based on these properties, onyx aligns much more closely with the definition of a stone rather than a crystalline material. The cryptocrystalline structure, banding, fracture pattern, and luster are key indicators that onyx should be classified as a stone.
What Causes the Banding in Onyx?
The distinctive banding that gives onyx its name is caused by trace amounts of iron oxide and other mineral impurities that are present during the formation process. As the silica solutions are depositing to form onyx, minor changes in the chemical composition cause color variations that result in the parallel bands.
The different colors of the bands are due to these impurities:
- Black bands – Rich in hornblende (amphibole mineral)
- White bands – Composed of aragonite or calcite
- Brown or red bands – Contain iron oxide
- Green or blue bands – Contain minerals like sodium sulfate or copper
The layered growth pattern of the cryptocrystalline silica allows these color variations to form in parallel lines or bands as the onyx is forming.
Onyx Formation and Geology
Onyx primarily forms in cave deposits from silica-rich solutions saturating the limestone bedrock. As the dissolved silica precipitates out of the groundwater solution, it forms successive layers of cryptocrystalline quartz that make up the banded onyx stone.
Onyx is found as fillings in lava gas cavities known as vesicles. These cavities form when gases get trapped in the lava and leave behind a hollow void. Silica-rich fluids permeate through the vesicles and deposit cryptocrystalline quartz in bands inside the cavity.
Major onyx deposits are found in Brazil, Uruguay, India, Madagascar, and in the United States. In the US, notable onyx mining locations include Arizona, California, Utah, and Texas.
Onyx Classifications
In gemology, there are several recognized subvarieties of onyx based on color banding:
- Black onyx – Parallel black and white bands
- Brown onyx – Brownish bands rather than black
- Red onyx – Parallel reddish or pinkish bands
- Green onyx – Alternating chlorite green bands
- Blue onyx – Layers containing blue sulfur or copper minerals
The most common and widely used type of onyx is black onyx with bold black and white banding. However, all varieties exhibit the cryptocrystalline structure and other physical properties of onyx stones.
Onyx Uses
Onyx has been used ornamentally and for jewelry for thousands of years. Some common uses of onyx include:
- Cabochons for rings, brooches, pendants
- Beads for necklaces and bracelets
- Handles and bases for cups or bowls
- Cameos and carvings
- Inlays and mosaics
- Intaglios for seals or engraving
- Vases, figurines, and statues
Black onyx is especially popular for men’s jewelry like rings and cufflinks. The stark contrast of black on white provides a bold, eye-catching look.
Onyx is also popular in home decorating for counter tops, backsplashes, and accents due to its layered pattern and polishable surface.
Historical and Spiritual Use of Onyx
Historically, black onyx was believed to have many mystical and protective powers. In ancient Greece and Rome, it was thought to ward off evil spirits, prevent nightmares, and bring good fortune. Greek brides wore black onyx jewelry to symbolize fidelity, while Roman athletes carried onyx amulets for extra strength and stamina.
The Bible references onyx stones multiple times. In the book of Exodus, onyx is described as one of the gemstones used for the vestments of the high priest Aaron. The twelve gemstone foundations of the New Jerusalem described in Revelation also include onyx.
Today, onyx remains popular in new age or mystical circles. It is believed by some to absorb negative energy, calm fears and worries, and promote wise decision making.
How Valuable is Onyx?
Compared to rarer colored gemstones like emeralds or sapphires, onyx is relatively affordable and available. However, quality can still vary pricing dramatically.
Some factors that determine onyx value include:
- Color – Bold, dramatic banding increases value. Solid black without white reduces value.
- Clarity – Free from cracks, inclusions, cloudiness
- Rarity – Some color varieties like green onyx are quite scarce
- Cut – Cabochons or beads that maximize banding patterns
- Carat Weight – Larger stones command higher prices per carat
Well-banded black onyx can cost $50 to $200 per carat. Rare types like blue onyx can reach $500 per carat. The most valuable onyx ever sold was a 224 carat red onyx cabochon that fetched over $70,000 at auction.
Onyx Lookalikes, Treatments, and Synthetics
There are a few minerals that resemble onyx and may be confused with or misrepresented as onyx.
Onyx Lookalikes
- Black agate – Very similar banding but has colored impurities between bands
- Schalenblende – Alternating calcite and hornblende rather than quartz
- Fluorite – Color banding but much softer than onyx
Onyx Treatments
To improve color and clarity, some onyx may be treated in various ways:
- Dying – Enhances or adds color to bands
- Waxing – Fills in cracks to improve clarity
- Sugar water or opticon treatments – Reduces visible flaws
These treatments are common, so buyers should verify that any very inexpensive onyx has not been altered from its natural form.
Synthetic Onyx
It is possible to produce synthetic onyx in laboratories, but it remains extremely rare. Synthetic onyx could replicate the banding and be passed off as real by dishonest sellers. Reputable gemological labs can identify these fakes through advanced testing methods.
Conclusion
In summary, onyx is classified as a stone rather than a crystalline gemstone. It exhibits the properties of stones like a cryptocrystalline structure, irregular fracture pattern, waxy luster, and parallel banding caused by impurities during formation.
Onyx has been used ornamentally for thousands of years, often for its mystical powers and bold black-and-white aesthetics. While not extremely rare or valuable, quality onyx remains popular in jewelry and home decor.