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Onyx Marble is a beautiful, often translucent, banded variety of calcite. It is characterized by its parallel bands of varying colors, which can include white, yellow, green, brown, and red, often with a waxy or vitreous luster. Despite its common name, it is not a true onyx (which is a cryptocrystalline quartz variety) nor a true metamorphic marble (which forms from the metamorphism of limestone). Instead, it is a form of travertine or flowstone, a chemical sedimentary rock composed of calcium carbonate. Its formation from mineral-rich waters gives it a unique appearance distinct from other carbonate rocks.
How to Identify
- Color
- Highly variable, often banded in shades of white, cream, yellow, green, brown, and red. Impurities like iron oxides (red/brown), chlorite (green), or organic matter (darker shades) contribute to the coloration.
- Luster
- Waxy to vitreous (glassy) on fresh surfaces.
- Texture
- Fine-grained to medium-grained crystalline, often with a sugary or granular appearance. Characterized by distinct parallel banding.
- Crystal Form
- Typically massive, forming layered or banded deposits. Individual calcite crystals are usually anhedral (lacking well-formed faces) within the mass, but can sometimes show rhombohedral cleavage.
- Cleavage
- Perfect rhombohedral cleavage (three directions not at 90 degrees), characteristic of calcite. This can be observed on broken surfaces.
- Geological Environment
- Forms in caves as speleothems (stalactites, stalagmites, flowstones) or at the mouths of hot or cold springs as travertine terraces. These environments are characterized by the precipitation of calcium carbonate from supersaturated groundwater or surface water.
Key Facts
- Hardness: 3 on the Mohs scale (can be scratched by a copper coin)
- Specific Gravity: 2.71 g/cm³
- Crystal System: Trigonal
- Color: Highly variable, often banded in white, cream, yellow, green, brown, red
- Luster: Waxy to vitreous
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect rhombohedral (three directions at 74°55')
- Composition: Calcium Carbonate (CaCO3)
Quick Check
- Color: Banded, variable (white, yellow, green, brown, red)
- Luster: Waxy to vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, banded, layered, stalactitic, stalagmitic, botryoidal
- Cleavage Type: Perfect rhombohedral (1011)
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Waxy to vitreous
Formation
Onyx Marble is a banded, often translucent, form of calcite (CaCO3) that precipitates from cold or hot spring waters or as speleothems (cave formations like stalagmites and stalactites). It is chemically identical to limestone and marble but differs in its formation process, which involves deposition from supersaturated aqueous solutions, leading to its characteristic banding and often crystalline texture. The banding results from variations in water flow, mineral content, and organic matter during deposition. It is not a true metamorphic marble in the geological sense, but rather a chemical sedimentary rock often referred to as travertine or flowstone, which can be subsequently subjected to low-grade metamorphism.
Usage
Primarily used as an ornamental and decorative stone in architecture for interior applications such as countertops, wall cladding, flooring, and decorative objects due to its attractive banding and translucency. It is also carved into sculptures and used in jewelry.
Age Distribution
Ranges from Holocene to Precambrian, depending on the specific deposit type (e.g., cave formations are typically younger, while some travertine deposits can be much older).
Where to Find
Mexico
Known for high-quality, often green and white banded Onyx Marble, particularly from Puebla and Oaxaca states.
Pakistan
Produces significant quantities of green and multi-colored Onyx Marble.
Iran
Historically and currently a major source of various colored Onyx Marble, often referred to as 'Oriental Alabaster'.
Turkey
Known for its travertine deposits, some of which are marketed as Onyx Marble, often in white, cream, and yellow tones.
USA
Deposits found in Arizona, California, and Utah, often associated with hot springs or cave systems.
Egypt
Ancient sources of 'Egyptian Alabaster' (which was actually Onyx Marble/travertine) used extensively in antiquity.
Finding Tips
Geological Context
Look for areas with active or ancient hot/cold springs, or limestone cave systems. Onyx Marble forms where calcium-rich waters emerge and degas CO2, leading to calcite precipitation.
Surface Features
Exposed deposits may show layered or terraced structures. In caves, look for stalactites, stalagmites, and flowstone formations.
Acid Test
As it is composed of calcite, Onyx Marble will effervesce (fizz) vigorously when a drop of dilute hydrochloric acid (HCl) is applied. This is a key diagnostic test to distinguish it from true onyx (quartz) or gypsum alabaster.
Translucency
Hold a thin piece up to a light source. Many varieties of Onyx Marble exhibit a degree of translucency, allowing light to pass through, especially in lighter-colored bands.
Similar Rocks
True Onyx
Chalcedony (variety of quartz)
Also known as: Sardonyx (if bands are white and reddish-brown)
Marble
Metamorphosed Limestone (primarily Calcite)
Also known as: Crystalline Limestone
Travertine
Calcite (chemically precipitated)
Also known as: Tufa (porous variety)
Alabaster (Gypsum)
Gypsum (CaSO4·2H2O)
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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