The diamond pickaxe can be equipped on a turtle. The pickaxe is the best tool for mining that can be equipped on a turtle. Because of the high cost of the pickaxe, it can mine anything and get the resources for it. The turtle can also attack with the pickaxe, and has the same damage output as a diamond shovel and diamond hoe.
For more information on the "Diamond Pickaxe", please go the the Minecraft official wiki.