sedimentary or igneous

Jasper and Basalt Mix (Dark Chert/Basalt River Pebble)

Silicon Dioxide (SiO2) or Mafic extrusive igneous rock

Hardness: 6-7 (Mohs scale), Color: Dark grey to charcoal black with slight mottling, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (Conchoidal fracture), Specific Gravity: 2.5-3.0

Identified April 22, 2026More sedimentary or igneous →

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Physical properties

Formation & geological history

Formed through either volcanic cooling (basalt) or chemical precipitation of silica (chert/jasper). This specific specimen shows signs of fluvial erosion, having been weathered and rounded in a river or stream environment over thousands of years.

Uses & applications

Common river stones are used primarily in landscaping, decorative aquariums, and occasionally for lapidary tumbling. In ancient times, such hard stones were used as tools or grinding stones.

Geological facts

Dark, smooth pebbles like this are often referred to as 'touchstones.' Historically, jewelers used them to test the purity of gold by observing the color of the streak left behind on the stone's surface.

Field identification & locations

Identify in the field by its smooth, water-worn texture, high density, and hardness (it should scratch glass). Common in riverbeds, glacial tills, and coastal beaches globally.

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 in Host Rock

Garnet Group (A3B2(SiO4)3)

metamorphic

Fluorite with Quartz coating

Calcium Fluoride (CaF2) with Silicon Dioxide (SiO2)

mineral

Opal

Hydrated Silica (SiO2·nH2O)

mineral