Alchemy stones are magical items in the Nyntindois universe. They work similar to Materia in Final Fantasy VII in that they get junctioned to weapons and armor to enhance them. They can also be occasionally junctioned to accessories as well. They have been used in Axem Adventure and War of the Worlds XI: Destiny's Call.
Alchemy stones are gems that are attuned alchemically to certain types of magic. For example, a Bolt-type stone makes the juntioned wielder able to cast some electric spells. However, some stones have non-magical abilities. For example, the Dual Wield stone gives the ability to wield two weapons.