Dragonstones

Dragonstones are red ruby-like stones that grow inside the brain of a dragon's earthly body. The dragonstone grows as the dragon ages, reaching a diameter of 4-6 feet before it stops growing. When the dragon is killed, the dragonstone provides an anchor that it can use to return to our world. The dragon's new body forms inside the dragonstone, which shatters when the dragon is reborn.
It's believed that possession of a dragonstone shard allows one to tap into the dragon's Ruh for magical purposes, which is why dragons closely guard the remains of their previous dragonstones. Being freely given a piece of a dragonstone is the greatest honor a dragon can bestow upon a human and happens very rarely. It is widely believed that Ilka the Faceless owes some of her power to a shard of dragonstone given to one of her ancestors by Kirlyok during the time of Dragon Pacts.
Kelek the Hunter was only able to permanently kill the dragons who protected the western cities because of powerful magic that somehow sundered the connection between the dragon and its dragonstone. Popular belief holds that imperial wizards are still performing rituals to keep the dragons from returning nearly 500 years after the last dragon was killed. As the dragons were killed, the stones were removed from their skulls and transported back to the capital. As the dragon cities fell to the empire, the dragonstones were returned to the city formerly protected by the dragon they belonged to and used in works of public art celebrating the glory of the The Eternal Empire. Kirlyok's last dragonstone was made into a pommel setting in the sword of the statue of Kyra Victorious.