Metamorphorical rock
The special, living, silicon-based tissue that Trolls are made from. It has the unique property of mimicking non-living stone around it: so that if a particular troll has a genetic affinity with granite, its body will take on the qualities, texture and colour of granite. (This also, of course, influences the name the young troll takes)
This genetic property of Trolls takes different turns in the big city: witness the young troll Brick, for instance, whose body is a sort of dull orange-red, patterned with regular lines in a paler colour. As we are also assured that in the Troll world, there is an unfortunate prejudice against Trolls whose metamorphorical shape takes a more sedimentary direction ("Chalky? I wouldn't give myself airs and graces if you could use me to write on a blackboard with, would you?") it is possible to see the young Brick is a victim of this social stigma - to another troll taking after a traditional material like marble or dolomite, a troll apparently made of mere fire-hardened clay would not be worthy of consideration. Earth? That's practically human!
Annotation
The word metamorphorical appears to be an amalgam or portmanteau of "metamorphic" (rock of igneous origin that has changed in structure due to time and pressure) and "metaphorical". It seems that the trolls' crusty exterior not only metamorphoses to suit its surroundings, but also provides a metaphor, or sometimes simply a pune on the character or occupation of the troll which is usually further indicated in the individual troll's name.
Examples:
- Tough, sharp, honest and always ready to help a compatriot in distress, Mr Shine is clearly "a diamond geezer".
- Detritus is rather rough and ready.
- Chalky is at the bottom of the social heap and does business with other races, he's poor white trash.
- Hardcore is an unrepentant criminal.
- Asphalt is a roadie.
- Chrysoprase is both hard and smooth, as befits a crimelord.
- Those little bits and pieces of the troll underworld who don't fit neatly into the Breccia form the DeBris Gang
- It is to be hoped that Detritus' new ex-scrape-head protege will grow up to show the kind of loyalty and dependability that would cause people of an earlier age to call him "a real brick".