Granodiorite (Gneissic Pebble)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Salt-and-pepper (gray, black, white/pink); Luster: Dull to vitreous; Crystal structure: Phaneritic (medium-grained); Cleavage: Dependent on mineral constituents (Quartz/Feldspar); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the slow cooling of magma beneath the Earth's surface. Often subjected to heat and pressure (metamorphism) which causes the banding or foliation visible in the specimen. Most commonly found in continental crust and batholiths.

Uses & Applications

Used primarily as crushed stone for road construction and aggregate. When polished, it is used for countertops, floor tiles, and grave markers similarly to granite.

Geological Facts

Granodiorite is the most common rock type in the Sierra Nevada Batholith of California. The famous Rosetta Stone was carved from a slab of granodiorite.

Field Identification & Locations

Identify in the field by its speckled 'salt-and-pepper' appearance, presence of visible quartz grains, and its extreme hardness (cannot be scratched by a steel blade). Look for rounded shapes in riverbeds or beaches where water erosion has polished the surface.

Identified on: 4/16/2026

Mode: Standard