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 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.