Irionite

Irionite — known in common speech as witchstone — is the green crystalline substance that is the indispensable fuel of high magic on Callidore. The word irionite is the Perwynese / scholarly term used by trained magi and the Arcane Orders; witchstone is what footwizards, hedgewitches, peasants, and most mundane folk call it. They are the same substance.

In its raw form a stone holds arcane power like a battery holds charge; bonded to a living caster it multiplies the mage's reserves many times over. Every High Mage in the series — Minalan, Pentandra, Tyndal, Rondal, and the rest — owes their rank to the possession of a bonded shard. Once bonded the stone becomes an extension of the mage's mind: it feels what they feel, stores power they pour into it, and cannot be easily taken from them without their death.

The irionite in active use was cleaved from the vast block of stone that contains the mind of Sheruel himself. Raw gurvani irionite is corrupt — using it risks linking the caster to Sheruel's will — so the Aronin of Amadia, and later the Alka Alon Council, developed the cleansing and bonding process that makes a stone safe for humani use. Minalan brings his first stone to the Aronin for cleansing early in the series; that pact is the debt that binds him to the Alka Alon cause.

The Spellmonger's Seven are the elite class: the seven Alka-crafted irionite spheres Minalan distributed to his most trusted lieutenants. Tyndal, Rondal, Gareth, and Iyugi all carry one. Most other High Magi carry stones derived from the shaman-stones Minalan captured, cleansed, and re-bonded through the Arcane Orders' licensing process.

The Royal Censorate of Magic spent four centuries trying to suppress irionite, destroying every sample they could find and burning out the Talent of any mage who possessed one. Their abolition, and the subsequent licensing of witchstones through the Arcane Orders, is the political and magical revolution at the heart of the series.