Opal's scratch hardness is measured at 6.0 to 6.5 on the Mohs' scale, similar in hardness to quartz , a little more than halfway between the hardness of talc and diamond .
(Opal's scratch hardness; is measured more; at 6.0 to 6.5)
Opal's scratch hardness
is measured more
at 6.0 to 6.5