@DrLeonJ @Aznorth @stux This is based on:

* Obviously, colour (#jasper is just "any red #chalcedony", chalcedony being semiprecious #flint/#chert - that is, cryptocrystaline #quartz)

* Availability in large sizes

* #Conchoidal fracture

* Brittle

Obviously I can't do any more tests than that from here!