Andesite is a fine-grained, extrusive igneous or volcanic rock. It is dark grey and made up of equal amounts of light and dark minerals, although the crystals are too small to be seen without a magnifier.
Occasionally andesite may contain some larger crystals. Or small round pockets that were gas bubbles.
How was it formed?
Andesite is a volcanic rock. It is fine-grained because it forms by the rapid cooling of magmas usually when it erupts onto the Earth’s surface and forms lava flows. Andesite forms from magma that contains less quartz (silica) than rhyolite but more than basalt. It is therefore often referred to as 'intermediate' in composition.
Andesite is the fine-grained extrusive equivalent of diorite.