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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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

Fluorite with Quartz coating

Calcium Fluoride (CaF2) with Silicon Dioxide (SiO2)

mineral

Opal

Hydrated Silica (SiO2·nH2O)

mineral

Healer's Gold (Pyrite and Magnetite in Schist)

Magnetite (Fe3O4) and Pyrite (FeS2) in Mica Schist

metamorphic

Red Garnet (likely Almandine or Pyrope)

Garnet group: X3Y2(SiO4)3 (e.g., Almandine: Fe3Al2Si3O12)

mineral