Diamond King

All trolls are made of metamorphorical rock. The Diamond King is a Troll born not of jasper or beryl or bluestone or granite, but pure diamond, and as such his destiny is kingship. As diamond trolls, they are capable of regulating the temperature of their brains by reflecting all heat. The troll brain being a lump of impure silicon, troll intelligence is lowered by increases in temperature. Diamond trolls' ability to regulate temperature makes them very intelligent, and intelligent in all environments.

It should be noted that, while trolls are supposed to be silicon-based life-forms, diamond actually consists of pure carbon (the element that we squashy people are based on) so really having diamond trolls is unscientific. However readers who are prepared to swallow the Discworld and its denizens probably don't care too much about a little thing like that.

The current Diamond King is Mr. Shine, who has spent most of his life living under cover in Ankh-Morpork, though he reveals himself at the end of Thud!. Another Diamond King brought the trolls to Koom Valley, and died in a cave playing Thud with B'hrian Bloodaxe after the "Battle", having recorded his voice on the Cube.