Basalt is a very common extrusive igneous rock. Basalt is the dominant rock in Earth’s oceanic crust. Black lava rocks seen at famous volcanoes such as Mt. Kilauea in Hawaii and Mt. Etna in Sicily are composed of basalt. Basalt has a mafic chemistry. Mafic igneous rocks are generally dark-colored, have 45-52% silica (= SiO2 chemistry), are rich in iron (Fe), magnesium (Mg), & calcium (Ca), and are dominated by the minerals plagioclase feldspar and pyroxene.
Basalts have an aphanitic texture (finely crystalline; all crystals <1 mm in size), but some are porphyritic or coarsely crystalline if the lava flow took time to cool & solidify (e.g., some Proterozoic basalt lavas in the Keweenaw Peninsula of Michigan's Upper Peninsula).
Olivine basalt (4.0 cm across) - some basalts have obvious yellowish-green olivine crystals (= ((Mg,Fe)2SiO4). This olivine basalt is from the Neogene near Keeler, western Inyo County, eastern California, USA.