Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Green Onyx is a translucent to opaque, banded form of calcite, characterized by its distinctive green hues, which can range from pale mint to deep forest green. It often exhibits parallel banding or swirling patterns, reflecting its depositional history. Despite its common name, it is not a true onyx (which is a variety of chalcedony, a cryptocrystalline quartz) but rather a type of marble composed predominantly of calcite. Its relative softness compared to true onyx makes it easier to carve and polish but also more susceptible to scratching and acid etching.
How to Identify
- Color
- Ranges from pale green to dark green, often with white, cream, or brown bands. The green color is due to impurities.
- Luster
- Vitreous (glassy) to waxy on fresh surfaces, can be dull if unpolished or weathered.
- Texture
- Fine-grained to medium-grained, often with a smooth, polished feel. Exhibits distinct banding or swirling patterns.
- Crystal Form
- Typically massive, microcrystalline to cryptocrystalline aggregates of calcite. Individual crystals are usually not visible to the naked eye.
- Cleavage
- Perfect rhombohedral cleavage (three directions at 74° and 106°), though often not apparent in massive forms unless fractured or stressed.
- Geological Environment
- Forms in karstic cave systems as speleothems (stalactites, stalagmites, flowstone) or as precipitates from mineral-rich hot springs and geysers (travertine deposits). Requires a source of calcium carbonate-saturated water and conditions for precipitation.
Key Facts
- Hardness: 3 (Mohs scale)
- Specific Gravity: 2.71 g/cm³
- Crystal System: Trigonal
- Color: Green (various shades), often banded
- Luster: Vitreous to waxy
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect rhombohedral (3 directions at 74° and 106°)
- Composition: Calcium Carbonate (CaCO₃) with impurities causing green coloration
Quick Check
- Color: Green, often banded with white/cream/brown
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, stalactitic, stalagmitic, columnar, granular, fibrous, lamellar
- Cleavage Type: Perfect rhombohedral
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to waxy
Formation
Green Onyx, or Onyx Marble, is a form of banded calcite that precipitates from cold or warm mineral-rich waters. It typically forms in caves as stalactites and stalagmites (speleothems), or as travertine deposits around hot springs and geysers. The green coloration is due to the presence of various impurities, most commonly chlorite, serpentine, or other iron-bearing silicates, or sometimes copper minerals, incorporated during its formation. The banding results from variations in water chemistry, flow rates, and impurity concentrations over time.
Usage
Primarily used as an ornamental stone for interior decoration, including countertops, wall cladding, flooring, sculptures, and decorative objects. Its translucent quality makes it popular for backlit applications. Also used in jewelry as cabochons or beads, though less common than true onyx (chalcedony).
Age Distribution
Varies widely, often Cenozoic, but can be found in older formations where suitable conditions for speleothem or travertine deposition existed.
Where to Find
Mexico
Significant deposits, particularly in Puebla and Oaxaca, known for producing high-quality green onyx.
Pakistan
Balochistan province is a major source of various colored onyx marbles, including green varieties.
Iran
Historical and contemporary source of onyx marble, often referred to as 'Oriental Alabaster'.
Turkey
Known for various types of travertine and onyx marble deposits.
Egypt
Ancient source of alabaster, which often included banded calcite varieties.
USA
Deposits found in Arizona, Utah, and California, often associated with hot spring activity or cave systems.
Finding Tips
Geological Context
Look for Green Onyx in areas with limestone bedrock, active or ancient cave systems, or around hot springs and geysers where calcium carbonate-rich waters are present or were historically active.
Surface Indicators
Outcrops may appear as banded, often translucent, masses within limestone or as layered deposits in stream beds or near spring orifices. Look for areas with evidence of water flow and mineral deposition.
Acid Test (Caution)
Calcite reacts with dilute hydrochloric acid (HCl), producing effervescence (fizzing). A small drop of 5-10% HCl on an inconspicuous part of the rock will confirm it is calcite. Always exercise caution and wear appropriate safety gear when handling acids.
Hardness Test
Calcite has a Mohs hardness of 3. It can be scratched by a copper coin (hardness 3.5) but will scratch a fingernail (hardness 2.5). This helps differentiate it from harder minerals like quartz (true onyx, hardness 7).
Similar Rocks
True Onyx
Banded Chalcedony (variety of Quartz)
Also known as: Chalcedony Onyx
Marble
Metamorphosed Limestone (primarily Calcite)
Also known as: Crystalline Limestone
Travertine
Porous Calcite (variety of Limestone)
Also known as: Tufa
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Calcite Group
- Crystal System
- Trigonal
- Chemical Formula
- CaCO₃
- Composition
- Calcium Carbonate, with trace impurities (e.g., chlorite, serpentine, iron oxides, copper minerals) responsible for green coloration.
Explore Green Onyx
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.