metamorphic
River Cobble (Phyllite or Quartzite)
Metasedimentary Rock (often containing SiO2 or KAl2(AlSi3O10)(OH)2)
Hardness: 6-7 (if quartz-rich); Color: Tan, grey, or greenish-brown; Luster: Dull to slightly silky (phyllitic sheen); Structure: Fine-grained, microcrystalline; Cleavage: Poor to slaty cleavage.
- Hardness
- 6-7 (if quartz-rich)
- Color
- Tan, grey, or greenish-brown
- Luster
- Dull to slightly silky (phyllitic sheen)
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Physical properties
Hardness: 6-7 (if quartz-rich); Color: Tan, grey, or greenish-brown; Luster: Dull to slightly silky (phyllitic sheen); Structure: Fine-grained, microcrystalline; Cleavage: Poor to slaty cleavage.
Formation & geological history
Formed through the regional metamorphism of sedimentary protoliths (like shale or sandstone). This specific specimen shows 'rounding' due to fluvial (river) erosion, indicating it was transported by water over a long distance.
Uses & applications
Commonly used in large-scale construction as aggregate, road base, or for decorative landscaping. Not typically used in jewelry due to its commonality and lack of refractive qualities.
Geological facts
The rounded, smooth edges suggest this rock has spent centuries in a riverbed. During the California Gold Rush, many such 'cobbles' were discarded by miners searching for the gold flakes often trapped in the crevices of riverbeds.
Field identification & locations
Identify in the field by its smooth, water-worn exterior and lack of visible crystals. It feels heavy for its size and will not scratch easily with a steel nail if it is quartzite-based. Found globally in current or ancient river systems.
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