Not 100% sure if this is an EMU glitch or not but. Get in a party and go mine something like Silver or Gold. Make sure the loot threshold is Uncommon and mine away. If you mine any other ore stack and hit into a gem, or mine gold/silver ore you get a roll dialog. Ignore that and you can keep mining the exact same mine stack and get mining levels for it, but in the end you only get whatever you get from 1 ore stack.
Tell me if this works on Live too, because if it doesn't I'll add an [EMU] tag to my post.
EDIT: Added [EMU] Tag