igneous and sedimentary

Banded River Cobble with Mafic Dike

Hydrothermally altered Graywacke or Sandstone with Mafic Intrusion

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs); Color: Tan/brown host rock with a dark gray/greenish band; Luster: Dull/earthy; Crystal structure: Mixed/Microcrystalline; Cleavage: None (conchoidal to irregular fracture)

Hardness: 6-7 (Mohs)
Color: Tan/brown host rock with a dark gray/greenish band
Luster: Dull/earthy

Identified April 19, 2026More igneous and sedimentary →

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Physical properties

Hardness: 6-7 (Mohs); Color: Tan/brown host rock with a dark gray/greenish band; Luster: Dull/earthy; Crystal structure: Mixed/Microcrystalline; Cleavage: None (conchoidal to irregular fracture)

Formation & geological history

Formed through the secondary intrusion of molten mafic material (like basalt) into a pre-existing sedimentary or metamorphic host rock, followed by millions of years of river transport and erosion which smoothed the edges into a cobble.

Uses & applications

Primarily used as decorative garden stones, in landscaping, or as geological teaching specimens to demonstrate cross-cutting relationships.

Geological facts

This specimen is a perfect example of 'cross-cutting relationships,' a geological principle stating that the geologic feature which cuts another is the younger of the two features.

Field identification & locations

Identify in the field by looking for distinct, straight-edged bands of a different color and texture cutting through a rock. Commonly found in riverbeds and high-energy alluvial deposits.