The “toughness” number, as I understand it, is a measure of brittleness.
This is a relative number. As you all know, by far the most common solid
mineral on earth is quartz with a hardness of 7. Therefore, all dust and
dirt contains matter of a hardness of 7 and will scratch softer material.
Cleaning eye glasses with a dirty cloth slowly etches it with quartz dust
as does cleaning softer There is a big difference between the
ability to scratch something (hardness) and the ability to break something
(brittleness or toughness).
I thought that the following table was going to be put up before this
earlier. I know that most of you have seen this list but some have not.
Please note the real authors listed below. I omitted the more unusual and
very brittle and/or soft stones.
Several surprising things can me shown from this table. If you accept this
table as correct, then garnets, zircons, typical emeralds, and topaz are
less suitable than iolite for rings.
TOUGHNESS AND HARDNESS FOR SOME GEM MATERIALS
(From Table 15 in “Faceting for Amateurs” by Vargas - Attributed to
Charles Parsons) Gem Material Toughness Hardness
10 Massonite* 10 9.25 Jadeite 8 6.5
Chrysoberyl 8 8.5 Corundum 8 9 Agate 7.5
7 Diamond 7.5 10 Quartz 7 7
Beryl 6.5 7.5 Tourmaline 6.5 7.5 Spinal
6.5 8 Peridot 6 6.5 Iolite 6
7 Garnet 5 to 6 7.5 Zircon 5.5
7.5 Emerald** 5.5 7.5 Topaz 5 8
Obsidian 5 5 Orthoclase 5 6 Andalusite
4.5 7.5 Opal 4 6
*Added from their advertisement
**For included emerald.