Something I came across today, apparently back in the 1970's a Florida Man (of course) sold off some thin strips of metal for refacing the strike plate of frizzens (for black powder rifles). Called D-38, it produced more sparks than a typical steel strike plate, allowing one to use even fairly-blunt flint to fire such weapons.
Unfortunately, it turns out
D-38 is a synonym for U-238, also known as depleted uranium. In addition to being mildly radioactive, DU is also extremely toxic if it gets into the body. Most people have the sense not to chew on chunks of uranium, of course (although
ceramics coated with uranium-based paint can be problematic), but you can't really help but breathe it in if it's been aerosolized - like, say, if someone decided to use it on the frizzen of a black powder rifle.
For gaming purposes, I could see an After the End scenario where people scrounge uranium (depleted or otherwise) from ancient battlefields and similar and use it in place of steel for fire starters, particularly if it's no longer known what it is (in which case, perhaps calling it something like "sparksteel" would work?). With how harsh the wastes can be, and with how much radioactivity may still be in play from the War (particularly if the setting is strongly inspired by
Fallout), it may take some time before anyone realizes the dangers of this material.
EDIT: This is probably cinematic, but in GURPS terms I could see a sparksteel strikeplate giving a +1 Malf for flintlock weapons and reducing the time needed to ignite a fire using flint and sparksteel in half; I forget if there's an FP cost for trying to start a fire with flint and steel, but if so using sparksteel should similarly cut this in half (as each attempt only takes half as long). Price would depend on the setting, but it's not going to be cheap (by weight anyway; you don't need much for a strikeplate or similar, so overall it might be relatively-cheap).