Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Plume Agate is a variety of chalcedony characterized by distinctive, feathery, or plume-like inclusions suspended within a translucent to opaque chalcedony matrix. The plumes can vary widely in color, including red, orange, yellow, brown, black, white, and sometimes green or blue, depending on the composition of the included minerals. The chalcedony matrix itself can be colorless, white, gray, or subtly tinted. The patterns often resemble moss, trees, or feathers, creating intricate and aesthetically pleasing designs. It is a microcrystalline form of quartz, meaning its individual crystals are too small to be seen without magnification.
How to Identify
- Color
- Matrix typically colorless, white, gray, or light brown; plumes can be red, orange, yellow, brown, black, white, green, or blue.
- Luster
- Waxy to dull on unpolished surfaces; vitreous (glassy) when polished.
- Texture
- Smooth to slightly granular on unpolished surfaces; conchoidal fracture surfaces are smooth.
- Crystal Form
- Cryptocrystalline, meaning individual crystals are microscopic and not visible to the naked eye. Forms botryoidal, mammillary, or stalactitic masses within cavities.
- Cleavage
- None. Exhibits conchoidal fracture.
- Geological Environment
- Primarily found in amygdaloidal cavities and veins within volcanic rocks (e.g., basalt, rhyolite, andesite) and sometimes in hydrothermal veins in other rock types. Also found as nodules in sedimentary deposits where silica-rich waters have permeated.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to chalcedony matrix).
- Specific Gravity: 2.58-2.64 g/cm³.
- Crystal System: Trigonal (for quartz/chalcedony).
- Color: Translucent to opaque matrix (colorless, white, gray, brown) with colored inclusions (red, orange, yellow, brown, black, white, green, blue).
- Luster: Waxy to dull (unpolished); vitreous (polished).
- Transparency: Translucent to opaque.
- Fracture: Conchoidal.
- Cleavage: None.
- Composition: Silicon dioxide (SiO₂) with various mineral inclusions (e.g., iron oxides, manganese oxides, chlorite).
Quick Check
- Color: Variable, but always with distinct feathery or plume-like inclusions.
- Luster: Waxy to vitreous.
- Streak: White.
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, often botryoidal, mammillary, or stalactitic, filling cavities. Inclusions form dendritic or feathery patterns.
- Cleavage Type: None.
- Fracture Type: Conchoidal, producing smooth, curved surfaces.
- Tenacity: Brittle.
- Luster Type: Waxy to dull on rough surfaces; vitreous when polished.
Formation
Plume Agate forms within cavities (geodes, amygdules, veins) in volcanic rocks (e.g., basalts, rhyolites, andesites) or, less commonly, in sedimentary environments. Silica-rich hydrothermal solutions, often carrying dissolved iron, manganese, or other trace elements, seep into these voids. As the solutions cool and evaporate, microscopic quartz crystals (chalcedony) precipitate in successive layers. The distinctive 'plumes' are formed by the inclusion of various mineral growths, typically iron oxides (hematite, goethite), manganese oxides, or other silicates, which crystallize in a dendritic or feathery pattern within the chalcedony matrix. The specific morphology of the plumes is influenced by the concentration of the included minerals, the rate of silica deposition, and the physical-chemical conditions of the environment.
Usage
Primarily used as an ornamental stone, particularly in lapidary arts for cabochons, spheres, carvings, and polished slabs. Its unique patterns make it highly sought after by collectors and jewelry designers. It is also used in decorative architectural elements and as a specimen for mineral collections.
Age Distribution
Varies widely depending on the geological setting of its formation, ranging from Cenozoic to Mesozoic and even older in some volcanic or sedimentary deposits.
Where to Find
Oregon, USA
Known for exceptional Plume Agate, particularly from areas like Prineville and Madras, often found in rhyolitic and basaltic flows. Varieties include 'Biggs Jasper' (a misnomer, it's a plume agate) and 'Graveyard Point Plume Agate'.
Mexico
Various localities produce Plume Agate, often with vibrant red and orange plumes, associated with volcanic activity.
Indonesia
Known for unique Plume Agate varieties, sometimes with intricate patterns and diverse colors, often associated with ancient volcanic activity.
Brazil
Produces various forms of agate, including some with plume-like inclusions, typically from basaltic flows.
Australia
Certain regions yield Plume Agate, often with earthy tones, found in volcanic and sedimentary contexts.
Finding Tips
Target Volcanic Terrains
Focus your search on areas with ancient or recent volcanic activity, particularly where basalt, rhyolite, or andesite are present. Look for weathered outcrops or stream beds that might contain eroded nodules.
Look for Geodes and Amygdules
Plume Agate often forms within these spherical or almond-shaped cavities. Look for rounded, often bumpy, rocks that feel heavier than typical host rock.
Check for Veins
Examine rock exposures for silica-filled veins, which can also host Plume Agate. These may appear as lighter-colored bands cutting through darker host rock.
Examine Weathered Surfaces
The outer rind of agate nodules can be dull and uninteresting. Look for areas where the rock has been broken or chipped to reveal the internal patterns. Carry a spray bottle with water to wet surfaces, which enhances visibility of patterns.
Safety Precautions
Always obtain permission before collecting on private land. Be aware of your surroundings, especially in rugged terrain. When breaking rocks, wear appropriate eye protection (safety glasses) and gloves. Be mindful of potential silica dust if cutting or grinding specimens, and use proper ventilation and respiratory protection (e.g., N95 mask) to prevent silicosis.
Similar Rocks
Dendritic Agate
Chalcedony with manganese oxide inclusions
Also known as: Tree Agate
Moss Agate
Chalcedony with chlorite or hornblende inclusions
Also known as: Mocha Stone
Ocean Jasper
Chalcedony with spherulitic inclusions
Also known as: Orbicular Jasper
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group (Chalcedony variety)
- Crystal System
- Trigonal
- Chemical Formula
- SiO₂ (with various trace elements and mineral inclusions)
- Composition
- Silicon dioxide, with inclusions typically composed of iron oxides (e.g., hematite, goethite), manganese oxides, or other silicates.
Explore Plume Agate
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.