Andesite

Andesite is a type of igneous rock.

Polished Andesite is the polished version of andesite. As with granite and diorite, it can be crafted from regular andesite.

World Generation
Regular andesite generates anywhere underground in rich seams, in an identical way to the other two stone types, as well as gravel and underground dirt. It is most commonly exposed to the surface in Extreme Hills biomes but can also be found regularly in other biomes.

Trivia
Along with Granite and Diorite, Andesite is the second block/item to be released in the Pocket Edition before the PC edition (not counting the developmental version). The first is Nether Quartz.
 * In real life, andesite is extrusive igneous rock of intermediate composition. Its intrusive counterpart is diorite.