Granodiorite (or Granite) Pebble

Physical Properties

Hardness: 6-7 (Mohs scale), Color: Mottled grey, tan, and black; Luster: Dull to greasy (when wet); Crystal Structure: Phaneritic (visible crystals of quartz, feldspar, and mafic minerals); Cleavage: None for the rock, but distinct for feldspar components.

Formation & Geological History

Formed through the slow cooling of magma deep beneath the Earth's surface (intrusive). The rounded edges suggest transportation via water (river or beach) where it underwent mechanical weathering.

Uses & Applications

Commonly used in crushed stone for construction, road base, and landscaping. As a polished pebble, it is often found in decorative garden displays or aquarium gravel.

Geological Facts

Granitoid rocks like this one make up the bulk of the Earth's continental crust. Because it contains both light minerals (feldspar/quartz) and dark minerals (biotite/amphibole), it is referred to as having a salt-and-pepper appearance.

Field Identification & Locations

Identify in the field by looking for interlocking crystals of different colors rather than a grain-by-grain sandy texture. It will not scratch with a pocket knife but will scratch glass.

Identified on: 4/26/2026

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