Basalt is fine-grained so that the individual minerals are not visible, but they include pyroxene, plagioclase feldspar and olivine. These minerals are visible in the coarse-grained, plutonic version of basalt called gabbro.
This specimen shows bubbles made by carbon dioxide and water vapor that came out of the molten rock as it approached the surface. During its long period of storage beneath the volcano, green grains of olivine came out of solution as well. So the bubbles, or vesicles, and the grains, or phenocrysts, represent two different events in the history of this basalt.
A big close-up picture of basalt is available as a free wallpaper image. See more basalts in the Basalt Gallery and learn much more in "Introducing Basalt."
Other galleries:
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society