igneous
Dark igneous rock, likely basalt or diabase, with a second purplish specimen
Basalt/diabase; exact identification is uncertain from the photograph
AI-generated identification · may be incorrectThe upper rock is dark gray-black, fine- to medium-grained, massive, and weakly lustrous; basalt has Mohs hardness about 6, while diabase is similar. The lower rock is coarse, purple-brown, and possibly weathered porphyritic igneous rock, but its minerals are not resolvable visually.
Identified More igneous →
Explore Dark igneous rock, likely basalt or diabase, with a second purplish specimen in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
The upper rock is dark gray-black, fine- to medium-grained, massive, and weakly lustrous; basalt has Mohs hardness about 6, while diabase is similar. The lower rock is coarse, purple-brown, and possibly weathered porphyritic igneous rock, but its minerals are not resolvable visually.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near the surface; diabase forms in shallow intrusions. These rocks commonly range from Precambrian to modern age, and the lower specimen may represent altered or weathered material.
Uses & applications
Basalt and diabase are widely used as crushed aggregate, road stone, riprap, paving, and occasionally building stone; attractive polished pieces are collected but are generally low-value.
Geological facts
Basalt is dominated by plagioclase and pyroxene, often with olivine or magnetite; diabase has similar chemistry but larger crystals. The purple coloration may reflect oxidation, alteration, or quartz/feldspar-rich material rather than a distinct gemstone.
Field identification & locations
Identify basalt by its dense dark color, fine grain, and lack of visible fossils; diabase shows interlocking crystals with a salt-and-pepper texture. A fresh surface, streak, magnetism, and hand lens would help distinguish them; common worldwide.
