Greetings and salutations.
Mister Surname, I presume? Ceana informed me of your adventures in Aleport. Terribly sorry about the misunderstanding. Though, as Professor Lamberteint is fond of saying, “No action taken in the name of science is ever truly wasteful.”
To the matter at hand─corrupted crystals possessing the properties you seek could be found in abundance at the Standing Corses following the Calamity.
Alas, that is no longer the case...and while there is at least one suitable crystal that remains to be found within the Twelveswood, precisely where it is at any given moment is more difficult to say.
Permit me to explain. You are familiar with spriggans, yes? The furry little rascals that are wont to seek out ores and other stones? Well, one such creature with an infamously insatiable appetite has single-handedly devoured every corrupted crystal in the Twelveswood. Fortunately for you, these crystals take an eternity to digest─assuming they can even be digested─and I am reasonably confident that one still resides within his gut.
It's quite fascinating, really. My theory is that this “Giggity” first consumed a corrupted crystal when... <sigh> You couldn't care less about all this, could you?
To the point, then. Giggity seldom makes his presence felt unless there is particularly fine fodder to be had. Therefore, we shall require a lure, and I have precisely the sort he will find impossible to resist.
Simply place this true heart─I shall save the story of the origin of its unusual name for another occasion─outside of the spriggans' lair and wait for the opportune moment to pounce.
As for the method by which you retrieve the crystal from his gut...I shall leave that to your discretion.