Here is a somewhat simple and efficient multi-level mining bot that can be set to ignore one type of block (usually stone). The bot will mine from the center and spiral outward to the user-defined radius. It will dig above or below it if it is not the block to ignore. If it is low on fuel, it will return to its origin and wait until it is refilled. If it is full, it will return to its origin and drop the items below it (put a chest under). Once it has finished a level and is not at the user-defined mining level, it will start to mine 3 block levels above it. The turtles status will always be shown while it is running.

Usage from lua header:
–Place block to ignore(usually stone) in first slot.
–Place fuel in second slot until requirement is met.
–Place a chest directly under the turtle.
–Run the program with block radius number, mining level, optional start radius, and optional start level.

http://pastebin.com/9kAtJP6j