http://en.wikipedia.org/wiki/Mohs_sc...neral_hardness
Though a decent reference, my wife likes to read in all sorts of places including a place known for it's sharp granulated
SiO2 crystals, and god, they get everywhere.
There are applications where extra protection makes sense, and some of us leave our kids in car seats till they weight 75lb or more... I like margining on the side of safety if it doesn't cost me much.