Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Green Onyx is a variety of banded calcite, a carbonate mineral. Despite its common market name, it is not true onyx, which is a cryptocrystalline quartz (chalcedony). It is characterized by its distinctive parallel bands of varying shades of green, often interlayered with white, yellow, or brown bands. The green color can range from pale mint to deep forest green. It is relatively soft compared to true onyx, with a Mohs hardness of 3. Its translucent to transparent nature allows light to pass through, enhancing its aesthetic appeal. It is composed primarily of calcium carbonate and reacts readily with acids.
How to Identify
- Color
- Ranges from pale mint green to deep forest green, often with white, yellow, or brown bands. The green is typically uniform within a band but varies between bands.
- Luster
- Vitreous (glassy) to waxy on fresh surfaces, can be dull on weathered surfaces.
- Texture
- Fine-grained to medium-grained, often with a smooth, polished feel. The banding is a key textural feature.
- Crystal Form
- Typically massive, microcrystalline to cryptocrystalline aggregates, forming stalactitic, stalagmitic, or layered structures. Individual calcite crystals are usually too small to be seen without magnification.
- Cleavage
- Perfect rhombohedral cleavage in three directions, though often not visible in massive, fine-grained specimens.
- Geological Environment
- Forms in karst environments (caves) as speleothems (stalactites, stalagmites, flowstone) or in hot spring deposits (travertine). It is a secondary mineral formed by the precipitation of calcium carbonate from groundwater or hydrothermal fluids.
Key Facts
- Hardness: 3 (Mohs scale)
- Specific Gravity: 2.71 g/cm³
- Crystal System: Trigonal
- Color: Various shades of green, often banded with other colors.
- Luster: Vitreous to waxy.
- Transparency: Translucent to transparent.
- Fracture: Conchoidal to uneven.
- Cleavage: Perfect rhombohedral in three directions at 74°55'.
- Composition: Calcium Carbonate (CaCO3).
Quick Check
- Color: Green, often banded with white, yellow, or brown.
- Luster: Vitreous to waxy.
- Streak: White.
Physical Characteristics
- Crystal Habit: Massive, stalactitic, stalagmitic, columnar, granular, fibrous, lamellar, or rhombohedral crystals (rarely macroscopic).
- Cleavage Type: Perfect rhombohedral.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy) to waxy.
Formation
Green Onyx, or banded calcite, forms through the precipitation of calcium carbonate (CaCO3) from cold or hot aqueous solutions. This process typically occurs in caves as stalactites and stalagmites (speleothems), or in hot springs as travertine. The banding, which gives it its 'onyx' appearance, results from variations in the mineral content, growth rate, and water chemistry during deposition. The green coloration is usually due to the presence of trace amounts of other minerals, such as chlorite, serpentine, or iron silicates, incorporated during its formation.
Usage
Primarily used as an ornamental and decorative stone in architecture, interior design, and sculpture. It is fashioned into tiles, countertops, sinks, decorative objects, and carvings. Its translucent quality makes it popular for backlit applications. It is also used in jewelry, though less commonly than true onyx (a chalcedony variety), due to its relative softness.
Age Distribution
Varies widely, from Cenozoic to Precambrian, depending on the specific geological formation and depositional environment.
Where to Find
Mexico
Known for significant deposits, particularly in the states of Puebla and Oaxaca, producing a wide range of colors including green.
Pakistan
A major source of green onyx, particularly from the Balochistan province, known for its vibrant green hues.
Iran
Historically a significant producer of 'onyx marble' (banded calcite), with various colorations including green.
Turkey
Contains numerous travertine and cave deposits that yield banded calcite suitable for ornamental use.
Egypt
Ancient quarries in Egypt produced 'Egyptian alabaster,' which is a form of banded calcite, sometimes with greenish tints.
USA
Deposits can be found in various states with karst topography or hot springs, such as Arizona, California, and Utah, though commercial production of green varieties is less prominent.
Finding Tips
Look for Karst Topography
Focus on regions with limestone bedrock, caves, and active or ancient hot springs, as these are the primary environments for banded calcite formation.
Identify Banding
The most distinctive feature is the parallel banding. Look for layers of varying color and translucency. This differentiates it from massive, unbanded calcite.
Perform an Acid Test (Carefully)
Calcite effervesces vigorously with dilute hydrochloric acid (HCl). A small drop will produce bubbles. This is a definitive test to distinguish it from true onyx (quartz), which does not react. Always wear appropriate safety gear and use a small, inconspicuous area.
Check Hardness
Green Onyx (calcite) has a Mohs hardness of 3, meaning it can be scratched by a copper coin or a steel knife. True onyx (quartz) has a hardness of 7 and cannot be scratched by these tools.
Observe Translucency
Many varieties of green onyx are translucent, allowing light to pass through, especially in thinner sections. Hold a thin piece up to a light source to observe this characteristic.
Similar Rocks
True Onyx
Banded Chalcedony (SiO2)
Also known as: Black Onyx, Sardonyx
Marble
Recrystallized Calcite (CaCO3)
Also known as: Calcite Marble
Travertine
Porous Calcite (CaCO3)
Also known as: Tufa
Serpentine
Magnesium Silicate (e.g., Antigorite, Lizardite)
Also known as: New Jade
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Calcite Group
- Crystal System
- Trigonal
- Chemical Formula
- CaCO3
- Composition
- Calcium Carbonate. Trace elements (e.g., iron, magnesium, manganese, copper, or organic matter) can substitute for calcium or be present as inclusions, influencing color. The green color is often due to chlorite, serpentine, or iron silicates.
Explore Green Onyx
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.