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A Soulstone is an item that can be used to unlock the Skills and memories of a Daemon. There is a different type of soulstone for each Daemon, usable only for that particular Daemon, and one generic type that can be used for any Daemon.

Soulstone Daemon
Sword Stone Aisha
Spear Stone Zaav
Farshot Stone Dia
Axe Stone Wil
Scythe Stone Izana
Fang Stone Zephyr
Fist Stone Lucika
Chain Stone Treize
Shieldcannon Stone Gavod
Twinblade Stone Rigan
Crystalcore Stone Jex
Null Stone Any

Weapon soulstones are received when battling enemies. Points are earned for using normal attacks, skill attacks, and Healing Incense; these points are not reported to the player. A soulstone is received each time the tally reaches a certain amount. The type of soulstone received matches the Daemon being used; for example a Sword Stone will be received if Aisha was used.

Null Stones are received for resolving a Lost Soul's problem, and can also be found in chests in the Beyond.