A refined stone of coalesced mana. Ingesting it gives +2 Max MP.
Value: 200g
Mana Stones increase mana capacity, but do not add mana. You will have to restore MP (e.g. using an Energy Potion) or simply proceed to the next day in order for your mana to be recharged to the new maximum.
Even after the summonable Slime learns the Morph ability, it will not have the requisite mana to do so while in the blue form. Using 3 Mana Stones on it will give it the necessary Max MP to use Morph.