Transmutation Stones
At the moment there’s nothing you can do with them. They’re account-bound, unsellable to vendors, and not usable in any MF recipe.
I’ve seen people lobbying for a promotion recipe that would convert Trans Stones > Crystals, but I doubt it’ll happen, TBH. Transmutation Crystals are probably a good source of continous income for Anet, and since you can start a new character and get a LOT of Transmutation Stones (you can get 18 without even leaving the major cities) I don’t think even a very high Stone requirement for a Crystal would cut it.
Best “use” you can have for them is probably going to be levelling a new character. Then you can use them like crazy for every appearence you’d possibly want.