I'm seeing a growing trend in the mine areas- people leaving the last nub of rock, or rocks partly mined. The problem is more common in the 'lower' rocks, but I've seen it happen to beryls and sparklies too.
The game does not respawn rocks unless they have been fully consumed, so please finish your rocks (and if you see leftover nubs, try to clear them as well). Besides, that last nub might have a diamond in it...