Weirstone

Weirstones are the magical stones that any member of any of the Magical Guilds is born with under their hearts. It is what gives them their power.

Powers
The Weirstone is passed down through family lines, but is not what makes a weir a weir. Jack Swift was 'weirflesh'  without a stone, causing him to begin to die, showing that a weir cannot survive without their stone. It has also been shown that dispite populat belive, the stone does not solely decide what kind of weir a weir is, as proven by Jack, who despite having a warrior stone, can still use High Magic, a power unique to wizards

As a Refuge
Through usage of a magical artifact known as a portal, weirstones can be used as a refuge, allowing the user to 'play dead'.

Weaknesses
A weirstone seems to work through it's connection to The Dragonheart, which can be broken, causing the weir to lose their powers without dying.

Linkage
A weir's stone can be linked to another's through an Old Magic ritual that requres their consent. This ritual allows weir to draw on eachother's powers and dominate any linked weir weaker than them.