igneous
Weathered Gabbro or Diorite (Igneous Cobble)
Phaneritic Basic/Intermediate Igneous Rock
Hardness: 6-7 on Mohs scale; Color: Salt and pepper (black/dark green and white) with brownish iron oxide weathering; Luster: Dull to sub-vitreous; Structure: Phaneritic (visible crystals); Specific Gravity: 2.7-3.3.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Salt and pepper (black/dark green and white) with brownish iron oxide weathering; Luster: Dull to sub-vitreous; Structure: Phaneritic (visible crystals); Specific Gravity: 2.7-3.3.
Formation & geological history
Formed from the slow cooling of magma deep beneath the Earth's surface. This specimen appears to be a stream-rounded cobble, likely of Cenozoic to Precambrian age depending on the local bedrock source.
Uses & applications
Crushed stone for road construction, railroad ballast, or polished decorative stone for countertops and memorials.
Geological facts
This rock exhibits a 'salt and pepper' appearance caused by the interlocking of light feldspar and dark ferromagnesian minerals like hornblende or pyroxene. The orange/brown patches are likely limonite result from chemical weathering.
Field identification & locations
Identify by its coarse-grained texture and heavy weight. Look for interlocking crystals of roughly equal size. Commonly found in riverbeds, glacial tills, or mountain ranges with exposed plutonic roots.
