The obsidian rock made into a well crafted blade. then doctors use it in cardiac surguery becuase it is so sharp even before it has been crafted. But if we used it for roads, we would have glassy...
Pumices hard surface is used for cleaning and polishing hope it helps! :)
They are used in tools because they are so hard.
Sculpture, facings, countertops, monuments; stuff like that usually.
So you're in the contest too huh? Granites is the answer for the cntraveler contest