Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Moss Agate is a translucent to opaque variety of chalcedony, a cryptocrystalline form of silica. It is characterized by distinctive dendritic (tree-like or moss-like) patterns of mineral inclusions, typically green, black, brown, or red, set within a colorless, white, or milky-white base. These inclusions are not organic but are mineral impurities, most commonly chlorite, manganese oxides, or hornblende. The patterns can resemble landscapes, foliage, or intricate abstract designs, making each piece unique.
How to Identify
- Color
- Base color is typically colorless, white, or milky-white, with green, black, brown, or reddish dendritic inclusions.
- Luster
- Waxy to vitreous (glassy) when polished, dull to waxy on rough surfaces.
- Texture
- Smooth to slightly granular on fracture surfaces; takes a high polish.
- Crystal Form
- Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. Forms in botryoidal, mammillary, or stalactitic masses, or as vein fillings and nodules.
- Cleavage
- None, exhibits conchoidal fracture.
- Geological Environment
- Found in volcanic rocks (e.g., basalts, andesites) and metamorphic rocks, often in geodes, nodules, or as vein fillings in areas of hydrothermal activity. It can also be found in alluvial deposits.
Key Facts
- Hardness: 6.5-7 on Mohs scale
- Specific Gravity: 2.58-2.64
- Crystal System: Trigonal (cryptocrystalline aggregates)
- Color: Colorless, white, or milky-white base with green, black, brown, or red dendritic inclusions.
- Luster: Waxy to vitreous
- Transparency: Translucent to opaque
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with inclusions of chlorite, manganese oxide, or hornblende.
Quick Check
- Color: Colorless to milky white base with green, black, brown, or red moss-like inclusions.
- Luster: Waxy to vitreous.
- Streak: White.
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, typically massive, botryoidal, mammillary, or as vein fillings and nodules.
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Waxy to vitreous
Formation
Moss Agate forms from silicon dioxide solutions that seep into cracks and fissures in volcanic or metamorphic rocks. As the silica-rich water slowly precipitates, microscopic quartz crystals (chalcedony) form. The distinctive 'mossy' or dendritic patterns are created by inclusions of various mineral oxides, primarily chlorite (green), manganese oxide (black/brown), or hornblende (dark green/black), which are trapped within the growing chalcedony. These inclusions are not organic plant matter, but rather inorganic mineral formations.
Usage
Primarily used as an ornamental stone, for cabochons, beads, carvings, and other lapidary purposes. Its unique patterns make it popular in jewelry. Historically, it has been used in folk medicine and as a talisman.
Age Distribution
Formed throughout geological history, often associated with volcanic activity and hydrothermal alteration.
Where to Find
India (Deccan Traps)
Historically a major source, particularly from the Deccan Traps region, known for high-quality green moss agate.
United States (Montana, Wyoming, Washington, Oregon)
Montana is famous for its 'Montana Moss Agate,' often found in alluvial deposits of the Yellowstone River, characterized by red and brown dendritic patterns. Other states also yield significant deposits.
Brazil
Produces a variety of agates, including moss agate, often with vibrant green inclusions.
Uruguay
Known for its agate deposits, including some moss agate varieties.
Australia
Various localities yield moss agate, often with unique inclusion patterns.
Russia
Deposits found in several regions.
Finding Tips
Look in Volcanic and Alluvial Areas
Moss Agate is commonly found in areas with past volcanic activity or in riverbeds and gravel deposits where it has been eroded from its original host rock.
Identify Host Rocks
Search for geodes, nodules, or vein fillings within basalt, andesite, or other igneous and metamorphic rocks.
Examine for Translucency and Inclusions
Look for translucent to semi-transparent stones with visible dendritic patterns. Wetting the surface can enhance visibility of these patterns.
Check for Hardness
Moss Agate is relatively hard (6.5-7 on Mohs scale), so it will scratch glass and steel. This helps differentiate it from softer minerals.
Similar Rocks
Dendritic Agate
Silicon dioxide (SiO2) with manganese oxide inclusions
Also known as: Tree Agate
Plume Agate
Silicon dioxide (SiO2) with various mineral inclusions
Also known as: Feather Agate
Chalcedony
Silicon dioxide (SiO2)
Also known as: Cryptocrystalline Quartz
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal (microcrystalline)
- Chemical Formula
- SiO2 (with various mineral inclusions)
- Composition
- Silicon dioxide with trace impurities forming dendritic patterns. The primary inclusions are typically chlorite (green), manganese oxides (black/brown), or hornblende (dark green/black). Iron oxides can also contribute to reddish or brownish hues.
Explore Moss Agate
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.