Minecraft wiki entry for Andesite, associated text: Andesite rock comes from above-ground lava flows. Finding andesite underground means the world's surface used to be lower, and the lava flow was buried.
Diorite is andesite's below-ground equivalent: The two rocks come from the same kind of magma, but diorite rocks are formed from solidifying underground magma pockets. The long, slow cool makes it look coarser than andesite.