Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Green Onyx is a beautiful, translucent to semi-translucent variety of banded calcite, characterized by its distinctive green hues and often parallel banding. Despite its common name, it is not true onyx, which is a cryptocrystalline quartz. Instead, it is a form of marble (metamorphosed limestone) or travertine, composed predominantly of calcium carbonate. Its attractive appearance and workability make it a popular material in architecture and decorative arts.
How to Identify
- Color
- Ranges from pale mint green to deep forest green, often with white, cream, or brown banding. The green color is typically uniform within bands but can vary between bands.
- 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. Banding is a key textural feature.
- Crystal Form
- Typically massive, microcrystalline to cryptocrystalline aggregates, forming banded layers. Individual calcite crystals are usually not visible to the naked eye.
- Cleavage
- Perfect rhombohedral cleavage (three directions, not at 90 degrees), though often not apparent in massive, fine-grained material unless broken or observed under magnification.
- Geological Environment
- Forms in karstic environments (caves, as speleothems like stalactites and stalagmites), around hot springs and geysers (as travertine), or in other settings where calcium-rich waters precipitate calcite. The green coloration indicates the presence of specific impurities during formation.
Key Facts
- Hardness: 3 (Mohs scale)
- Specific Gravity: 2.71 g/cm³
- Crystal System: Trigonal
- Color: Various shades of green, often banded
- Luster: Vitreous to waxy
- Transparency: Translucent to semi-translucent
- Fracture: Conchoidal to uneven
- Cleavage: Perfect rhombohedral (3 directions at 74°55')
- Composition: Calcium Carbonate (CaCO3)
Quick Check
- Color: Green, often banded with white/cream
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, microcrystalline to cryptocrystalline aggregates, often forming banded layers (speleothems, travertine).
- Cleavage Type: Perfect rhombohedral, typically not observed in massive material unless fractured.
- Fracture Type: Conchoidal to uneven, splintery.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy) to waxy, sometimes dull.
Formation
Green Onyx is a form of banded calcite, which is a chemical sedimentary rock. It forms from the precipitation of calcium carbonate (CaCO3) from cold or hot aqueous solutions. This typically occurs in caves as stalactites and stalagmites, or in hot springs as travertine. The banding is a result of variations in the rate of deposition, water chemistry, and the presence of impurities during formation. The green coloration is usually due to the presence of trace amounts of minerals like chlorite, serpentine, or other iron-bearing silicates, or sometimes copper compounds, incorporated during its growth.
Usage
Primarily used as an ornamental stone for interior decoration (countertops, wall cladding, flooring, decorative objects), sculptures, and sometimes in jewelry (cabochons, beads). Its translucent quality makes it popular for backlit applications.
Age Distribution
Can form in various geological periods, from Precambrian to Cenozoic, wherever suitable conditions for calcite precipitation exist.
Where to Find
Mexico
Known for significant deposits, particularly in the state of Puebla, producing high-quality Green Onyx often referred to as 'Mexican Onyx'.
Pakistan
A major source of Green Onyx, particularly from the Balochistan province, known for its vibrant green varieties.
Iran
Historically and currently a significant producer of various onyx marbles, including green varieties.
Turkey
Known for travertine and onyx marble deposits, some of which exhibit green coloration.
Egypt
Ancient quarries in Egypt yielded 'Oriental Alabaster' which included banded calcite, some with green hues.
USA
Limited deposits of banded calcite (travertine/onyx marble) can be found in states like Arizona, California, and Utah, though green varieties are less common than in other regions.
Finding Tips
Geological Context
Look for Green Onyx in areas with limestone bedrock, particularly near caves, hot springs, or ancient spring deposits. These are environments conducive to calcite precipitation.
Visual Identification
Identify by its characteristic green color, often with parallel banding of varying shades of green, white, or cream. Its translucency is a key feature.
Hardness Test
Perform a scratch test. Calcite has a Mohs hardness of 3, meaning it can be easily scratched by a copper coin (hardness ~3.5) or a steel knife (hardness ~5.5). This distinguishes it from true onyx (quartz, hardness 7).
Acid Test (Caution Recommended)
A small drop of dilute hydrochloric acid (HCl) will cause calcite to effervesce (fizz) due to the release of carbon dioxide. This is a definitive test for calcite but should be done carefully on an inconspicuous area as it can damage polished surfaces.
Specific Gravity
While not a field test, its specific gravity (2.71 g/cm³) can help differentiate it from other minerals if laboratory equipment is available.
Similar Rocks
True Onyx
Chalcedony (SiO2)
Also known as: Sardonyx (if banded with red/brown)
Marble
Calcite (CaCO3)
Also known as: Crystalline Limestone
Travertine
Calcite (CaCO3)
Also known as: Tufa (less dense, porous variety)
Serpentine
Mg3Si2O5(OH)4
Also known as: Bowenite, Antigorite, Lizardite
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 compounds) responsible for green coloration.
Explore Green Onyx
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.