Granodiorite Pebble

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Salt and pepper (mottled grey, white, and black). Luster: Dull/Matte (tumbled). Crystal Structure: Phaneritic (medium-grained visible crystals). Cleavage: Varies by mineral constituent. Specific Gravity: 2.6-2.7.

Formation & Geological History

Formed through the slow cooling of silica-rich magma deep underground (intrusive). This specific specimen has been tumbled and rounded by water action, likely in a river or beach environment. Age can range from millions to billions of years depending on the batholith of origin.

Uses & Applications

Primary use is in construction (crushed stone, road base) and architecture (countertops, curbing). Small water-worn pebbles are used decoratively in landscaping or aquariums.

Geological Facts

Granodiorite is intermediate between granite and diorite. The famous Rosetta Stone was carved into a slab of granodiorite, not basalt as was originally thought.

Field Identification & Locations

Identify by its 'salt and pepper' appearance and visible crystals. It contains more plagioclase feldspar than orthoclase feldspar compared to true granite. Commonly found in mountainous regions or glacial till and riverbeds downstream from mountain ranges.

Identified on: 4/24/2026

Mode: Standard