Basalt is a common extrusive volcanic rock (usualy that means lava). It is usually grey to black and fine-grained due to rapid cooling at the surface of a planet. It may be porphyritic (containing larger crystals in a fine matrix), or vesicular (has gas bubbles). Unweathered basalt is black or grey. According to the official definition, basalt contains, by volume, less than 20% quartz and less than 10% feldspar.

On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt has also formed on Earth's Moon, Mars,Venus, and even on the asteroid Vesta. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.

The word "Basalt" is derived from Late Latin basaltes, misspelling of L. basanites "very hard stone," which was imported from Ancient Greek basani'ty*s (basanites), from ba'sano*s (basanos, "touchstone").

Basalt is used in construction (e.g. as building blocks or in the groundwork), making cobblestones (from columnar basalt) and in making statues. Heating and extruding basalt yields stone wool, an excellent thermal insulator.

 

For more information try these links.

http://stoneplus.cst.cmich.edu/basalt.htm

http://geology.com/rocks/basalt.shtml

http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry27.html

http://csmres.jmu.edu/geollab/fichter/IgnRx/Basalt-1A1.html