sedimentary or igneous

River Pebble (Greywacke or Basaltic)

Massive Greywacke or Basaltic Andesite

Hardness: 6-7 (Mohs); Color: Dark grey to charcoal; Luster: Dull/Matte; Structure: Microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: approx. 2.7-3.0

Hardness: 6-7 (Mohs)
Color: Dark grey to charcoal
Luster: Dull/Matte

Identified April 6, 2026More sedimentary or igneous →

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Physical properties

Hardness: 6-7 (Mohs); Color: Dark grey to charcoal; Luster: Dull/Matte; Structure: Microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: approx. 2.7-3.0

Formation & geological history

Likely formed through rapid volcanic cooling or underwater turbidity currents. The rounded shape indicates mechanical weathering and transportation in a high-energy river or beach environment.

Uses & applications

Common river stones are used for landscaping, construction aggregate, aquarium decor, and primitive stone tools.

Geological facts

River pebbles reach their smooth, rounded shape via a process called abrasion, where water-borne rocks collide with one another and the riverbed over many years.

Field identification & locations

Identify by its water-worn rounded edges and uniform fine-grained texture. Hard enough to scratch glass. Commonly found in riverbeds, glacial deposits, and coastal shorelines.

Other sedimentary or igneous specimens

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Pyrite

Iron Sulfide (FeS2)

mineral

Pebble (likely a type of sandstone or quartzite pebble)

Clastic Sediment (specifically, a rudite-sized clast). If the pebble is composed of a specific rock type, its scientific name would be that rock type (e.g., 'Quartzite pebble', 'Granite pebble', 'Chert pebble'). Given its appearance, it is likely a metamorphic quartzite pebble or a hard sandstone pebble.

Sedimentary rock

Quartz Vein in Host Rock

Silicon Dioxide (SiO2) within Sedimentary or Igneous Matrix

igneous

Garnet

Garnet (X3Y2(SiO4)3)

mineral

Gneissic Schist

Gneiss-schist (Metamorphic rock)

metamorphic

Garnet in Host Rock

Garnet Group (A3B2(SiO4)3)

metamorphic