igneous
Basalt with Chert/Obsidian blade
Aphanitic Mafic Igneous Rock (Basalt) with Microcrystalline Silica (Chert)
Hardness: 5-7 Mohs; Color: Dark grey to reddish-brown matrix with a black glassy fragment; Luster: Dull/earthy matrix with vitreous/waxy fragment; Cleavage: None (conchoidal fracture on the dark flake)
- Hardness
- 5-7 Mohs
- Luster
- Dull/earthy matrix with vitreous/waxy fragment
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Physical properties
Hardness: 5-7 Mohs; Color: Dark grey to reddish-brown matrix with a black glassy fragment; Luster: Dull/earthy matrix with vitreous/waxy fragment; Cleavage: None (conchoidal fracture on the dark flake)
Formation & geological history
Basalt forms from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. The dark, sharp fragment appears to be a separate lithic flake (like chert or obsidian) resting on or fused to a lateritic basalt crust.
Uses & applications
Basalt is used extensively in construction for aggregate and road base. Sharp silica-rich rocks like the black fragment were historically used for stone tools.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The reddish hue indicates oxidation (weathering) of iron minerals, often referred to as 'scoria' or 'laterite'.
Field identification & locations
Identify by its fine-grained texture, dark color, and heavy weight relative to size. The black shard shows diagnostic conchoidal (shell-like) fracturing, which is typical of volcanic glass or chert.
