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.
Normally you would like to saty at 7.5 to 8 and above in hardness and
toughness for gems that are going into rings.
Thank you,
Lance Kanaby
GemData, Mine Design & The Spectrum Network at http://www.gemdata.com
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