A rare and sought-after magical metal which needs to be handled with care, as only a very small amount (far less than is needed to make a dagger blade) builds to an unstable critical mass. It appears to originate in the Loko Region of Uberwald.
Usually jet-black, the metal is sought by rich Assassins to make signature rings. Vetinari's official seal of office is made of stygium, and carries only a plain "V" to be impressed into black sealing wax.
While the small amount needed for a ring is usually stable, care needs to be taken about exposing it to daylight as this can cause it to flash to white-heat.
Vetinari's seal-ring is normally kept in a black velvet-lined box, and only brought out at need. Other wearers, such as non-practising assassin Cosmo Lavish, will affect wearing their stygium ring underneath a black glove for safety reasons. Cosmo should have also taken into consideration that an otherwise 100% exact replica of Vetinari's ring should have been resized to safely fit his far larger finger... this would have been far more to the point as a safety factor and spared him blood poisoning, gangrene, near-death and a permanently missing ring finger...
As with the Low King and the Scone of Stone, Vetinari has used this ring as a method of validating the truth (or otherwise) of what he is told by a subordinate he suspects of being economical with the facts. Moist von Lipwig passed the test, but only just...
Interviewed at the end of the The New Discworld Companion, Terry Pratchett talks about rings. He says that for quite some time, he had the one-and-only unique Death's Head ring in gold (the manufacturers made all the rest in silver) and was content with that, until the day came when the manufacturers were talked into doing a limited issue in gold. Terry now sports the one, single, unique example of the ring in platinum, but is resigned to the moment when this will no longer be unique. He is putting thought into the metal for his next ring, looking for an element that will make it truly unique. He has considered titanium, but the idea of mercury has crossed his mind, although he realises the necessary drawback would be having to store the otherwise unwearable ring in an industrial freezer. But it would most probably remain unique.
Perhaps contemplating a ring, made of a material that would freeze-burn an entire finger off, provided the jumping-off point that led to stygium?
- Hematite is a black, lusterous mineral and makes lovely jewelery.
(Ref. The New Discworld Companion, The Terry Pratchett Interview: Discworld quo Vadis? , p471 (Gollancz PB))