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Onyx Marble is a translucent to opaque, banded variety of calcite (calcium carbonate). It is characterized by its distinctive parallel bands of varying colors, often white, cream, yellow, green, brown, and red. Despite its common name, it is not true onyx (which is a cryptocrystalline quartz variety) nor is it a metamorphic marble (which is recrystallized limestone). It is a chemical sedimentary rock, specifically a type of travertine, distinguished by its often finer grain size and more pronounced banding compared to other travertines.
How to Identify
- Color
- Highly variable, often white, cream, yellow, green, brown, red, or a combination, arranged in distinct parallel bands.
- Luster
- Vitreous to waxy, often duller on unpolished surfaces.
- Texture
- Fine-grained to microcrystalline, often with a sugary or granular feel on broken surfaces. Polished surfaces are smooth.
- Crystal Form
- Typically massive, banded, stalactitic, or stalagmitic. Individual calcite crystals are usually microscopic.
- Cleavage
- Perfect rhombohedral cleavage (three directions not at 90 degrees), though rarely observed in massive forms.
- Geological Environment
- Forms in karstic environments, caves (as stalactites, stalagmites, flowstone), and around hot or cold springs where calcium-rich waters degas carbon dioxide, leading to the precipitation of calcium carbonate.
Key Facts
- Hardness: 3 on Mohs scale (can be scratched by a copper coin)
- Specific Gravity: 2.71 g/cm³
- Crystal System: Trigonal
- Color: Varied, often banded (white, cream, yellow, green, brown, red)
- Luster: Vitreous to waxy
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect rhombohedral (3 directions at 74° 55')
- Composition: Calcium Carbonate (CaCO₃)
Quick Check
- Color: Banded, various colors (white, cream, yellow, green, brown, red)
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, banded, stalactitic, stalagmitic, botryoidal, reniform
- Cleavage Type: Rhombohedral, perfect on {1011}
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to waxy
Formation
Onyx Marble is a form of travertine, a terrestrial sedimentary rock, formed by the precipitation of carbonate minerals from solution in fresh water, typically at the mouth of hot springs or cold springs, or in caves. The banding, which gives it its 'onyx' appearance, results from variations in water chemistry, flow rate, and the presence of impurities (e.g., iron oxides for reds/yellows, organic matter for browns/blacks) during deposition. It forms in layers as water evaporates or degasses CO2, causing calcium carbonate to crystallize.
Usage
Primarily used as an ornamental and decorative stone in architecture for interior applications such as countertops, wall cladding, flooring, and decorative objects (vases, sculptures). Its translucency makes it popular for backlit installations. Historically, it was used for carvings and statuary by ancient civilizations.
Age Distribution
Ranges from Holocene to Pliocene, depending on the specific deposit, as it forms in active or recently active spring systems.
Where to Find
Mexico
Significant deposits, particularly in Puebla, producing a wide range of colors and patterns, often referred to as 'Mexican Onyx'.
Pakistan
Known for producing green and multi-colored onyx marble.
Iran
Historical source, particularly for yellow and white varieties.
Turkey
Various deposits yielding different colors and patterns.
Egypt
Ancient source, often called 'Egyptian Alabaster' historically, used extensively in pharaonic times.
USA
Deposits in Arizona, California, and Utah, often associated with hot springs.
Finding Tips
Look for Karstic Features
Onyx Marble is found in areas with limestone bedrock, caves, and active or extinct hot/cold springs. Look for evidence of past or present water flow and calcium carbonate deposition.
Identify Banding
The most distinctive feature is its parallel banding. This differentiates it from unbanded travertine or other massive calcite deposits.
Acid Test (Caution)
Calcite reacts vigorously with dilute hydrochloric acid (HCl), producing effervescence (fizzing) due to the release of CO2. This test should be done carefully on a small, inconspicuous area as it can damage polished surfaces. This distinguishes it from true onyx (quartz), which does not react.
Check for Translucency
Many varieties of Onyx Marble exhibit a degree of translucency, especially in thinner sections, which can be observed by holding a light source behind it.
Similar Rocks
True Onyx
Cryptocrystalline Quartz (Chalcedony variety)
Also known as: Sardonyx (if bands are red/brown and white)
Marble
Metamorphosed Limestone/Dolomite
Also known as: Recrystallized Limestone
Travertine
Porous Calcite
Also known as: Tufa (if porous)
Alabaster (Gypsum)
Fine-grained Gypsum
Also known as: Gypsum Alabaster
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Calcite Group
- Crystal System
- Trigonal
- Chemical Formula
- CaCO₃
- Composition
- Calcium Carbonate
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