Iolite has a Moh’s hardness of 7, but only 6 on a toughness scale.
Hardness is the ability scratching, while toughness is the wearability of
the gem material.
I have never heard of a toughness scale and would like to get a copy of
one. Is it listed anywhere on the net? In Oppi’s book?
Jill Alessandra Jewelry