Sergeant Detritus

Trolls in Terry Pratchett's Discworld novels, unlike the monstrous trolls of folklore and J. R. R. Tolkien, have been subverted into a moderately civilised race. Trolls on the Discworld are, essentially, living, mobile rocks. Trolls have grown to overcome those vicious stereotypes of yore and have lived very prosperous lives in heavily populated cities with (relatively) little killing, and they have held jobs as diverse as police officer and concert promoter. They have also held jobs as "bridgekeepers", collecting tolls from those that cross their bridges. It is relatively harmless, although farmers in the company of billy goats have to pay a hefty toll.

Unlike the flesh and blood Troll of Scandinavian folklore that turn to stone only when exposed to daylight, Disc trolls are stone all the time, but become dormant and sluggish during daylight. Trolls are made of "metamorphorical rock" (a play on metamorphic rock and metaphorical), which means that trolls take on some of the appearance and characteristics of certain stones and minerals, after which they are usually named. The most extreme example of this is the pitiful troll Brick in the novel Thud!, Ankh-Morpork born and bred, who took on the appearance of the man-made bricks of his city home. Some trolls are made of sedimentary rock and trolls who have taken the forms of more solid minerals are sometimes prejudiced against their sedimentary kin. On very rare occasions, a troll made of diamond is born. These trolls are vastly more intelligent than most trolls, as their reflective bodies allow them to ward off heat and regulate their internal temperature. A diamond troll is the indisputable king of the trolls, whether it wants to be or not.

The common troll is nocturnal. Their natural tendency to spend daylight hours dormant led to the stories of them turning to stone when hit by the sun. Though apparently unintelligent, this is due to heat negatively affecting the conductivity of their silicon brains, which operate in a similar fashion to computer circuits (in essence a trollish form of hyperthermia). In the coldness of the high mountain areas that they usually call home, they can, in certain respects, be quite clever. When sufficiently deep-frozen, they can perform exceptional mathematical feats, possibly involving quantum computing. Oddly, the silicon-based trolls are capable of freezing to death if exposed to sufficiently cold temperatures, although the book Men at Arms makes it clear that such temperatures are far lower than fleshly creatures, such as dwarfs, could survive.

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Wikipedia