igneous
Basalt (Beach Pebble) / Black Chert
Basalt / Microcrystalline Quartz (SiO2)
Hardness: 6-7 on Mohs scale. Color: Dark grey to pitch black, wet/glossy appearance. Luster: Vitreous to dull when dry. Structure: Fine-grained (aphanitic) or cryptocrystalline. Cleavage: None, conchoidal fracture.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to pitch black, wet/glossy appearance
- Luster
- Vitreous to dull when dry
Identified More igneous →
Explore Basalt (Beach Pebble) / Black Chert in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark grey to pitch black, wet/glossy appearance. Luster: Vitreous to dull when dry. Structure: Fine-grained (aphanitic) or cryptocrystalline. Cleavage: None, conchoidal fracture.
Formation & geological history
Formed through the rapid cooling of mafic lava at or near the Earth's surface, or via biochemical precipitation of silica (if chert). Rounded and smoothed over time by river or ocean wave action.
Uses & applications
Commonly used as aggregate in construction, road base, decorative landscaping stones, and tumbling material for rockhounds.
Geological facts
Water-rounded black pebbles found on beaches are often basalt, chert, or obsidian. When wet, light reflects strongly off their smooth, wave-worn surfaces.
Field identification & locations
Look for a smooth, well-rounded shape with fine grain and a deep dark grey or black color. Frequently found along coastal beaches, riverbeds, and volcanic regions. Hard enough to scratch glass.
