We use them to channel our spells. The large ones you'd normally just hold, but the little ones are more common. Usually, those would be in an accessory we have on our person, like an earring or ring or hairclip -- or something.

[Anna shakes her head at "hairclip" as if she's trying to get something out of her head. She doesn't seem to have any accessory with a stone on her like she's describing.]

Inside the illusory world during GRANBELM, they're mirrored as the core of the mage's ARMANOX. Break the core, shatter the stone, since the two are connected.