igneous
Dark fine-grained rock fragment
Probably basalt or basaltic scoria; visual identification is uncertain
AI-generated identification · may be incorrectBlack to dark gray, fine-grained and dull, with irregular fracture; likely hardness about 5–6 Mohs and specific gravity around 2.7–3.1. No clear crystals, cleavage, or diagnostic vesicles are visible at this scale.
Identified More igneous →
Explore Dark fine-grained rock fragment in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Black to dark gray, fine-grained and dull, with irregular fracture; likely hardness about 5–6 Mohs and specific gravity around 2.7–3.1. No clear crystals, cleavage, or diagnostic vesicles are visible at this scale.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic flows; most examples are geologically young to ancient. The fragment may also be weathered industrial rock, so testing is needed.
Uses & applications
Used as crushed aggregate, road base, concrete aggregate, riprap, and occasionally as landscaping stone; ordinary fragments have negligible jewelry value.
Geological facts
Basalt is the most common volcanic rock and makes up much of oceanic crust. Its dark color reflects abundant pyroxene, plagioclase, and commonly olivine.
Field identification & locations
Look for tiny interlocking crystals, a dark streak-free surface, and occasional gas bubbles; it should not fizz in dilute acid. A hand lens, streak test, magnet test, and fresh broken surface would improve identification; common worldwide, especially volcanic regions.
