If you don't have time and skills for the Battlegrounds but have enough to spare for killing Sleepers for their Great Natures(25% drop).
You can exchange 8 of those for a Flame Heart or a Mystic Frozen. 12 for a Rough Wind.
The NPC is at Amatsu Palace.
I do hope that WarpPortal will add an option for "Excange All Stones".
The current option only gives you one stone per transaction.