I'm not sure what this stuff is but our front garden beds are full of it, sold as "rainbow stone". I assume it's some kind of quartz. Pretty fiddly with it being so small but thought I might as well give it a shot, and can get sparks off a 1084 knife spine but no ember yet. Might make a little striker next time I fire up the 2 brick forge.
It does chip pretty easily but there's plenty more where it came from!