River Cobble (Quartzite / Gneiss)
Quartzite or Quartzo-feldspathic Gneiss · Mineral Specimen / Decorative Stone
Crystal System
Trigonal (Quartz host) within a Metamorphic fabric; Habit is massive, heavily rounded by fluvial erosion (cobbily/alluvial)
Mineral Group
Silicates (Tectosilicates)
Chemical Formula
SiO2 (predominantly), usually with KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8 and various mica/amphibole inclusions
Formation Process
Metamorphic: Likely formed through regional metamorphism of sandstone (Quartzite) or granite (Gneiss), later weathered and shaped by high-energy water transport
About This Crystal
An opaque, rounded river stone (cobble) showing a mottled gray, cream, and dark purple/brown coloration. It exhibits a dull/earthy luster due to weathering, with a somewhat granular to foliated texture consistent with metamorphic rock.
Physical Characteristics
Color: Gray to dark purple; Streak: White; Cleavage: None (fractures across grains); Fracture: Uneven/Sub-conchoidal; Specific Gravity: ~2.6-2.7; Tenacity: Brittle to Tough; Fluorescence: Generally none.
Optical Properties
Opaque; Non-pleochroic; Index of Refraction: ~1.54-1.55 (grains); No specific optical phenomena like asterism or chatoyancy visible in this weathered state.
Hardness & Durability
Origin Region
Found globally in riverbeds, glacial tills, and alluvial deposits; likely sourced from a regional metamorphic mountain belt
Hardness & Durability
Mohs Hardness: 6.5 to 7; High toughness due to interlocking crystalline structure; Very stable and resistant to chemical weathering.
Care & Maintenance
Extremely durable; can be cleaned with warm soapy water and a brush. Resistant to sunlight and most household chemicals.
Rarity & Value
Very Common; Low monetary value as a single specimen. Value is primarily aesthetic or as a decorative 'worry stone'.
Special Characteristics
Shows distinctive fluvial erosion (rounding) and internal banding/foliation typical of medium-grade metamorphic rocks.
Lore & History
Historically used as grinding stones, building material, or simple tools (hammerstones). In modern crystal lore, river stones are often associated with the element of water and the concept of 'going with the flow' or endurance through time.