It is often valuable to know the degree of hardness of different
substances, and these can easily be remembered by the following list.
In degree of relative hardness the list is as follows, the hardest
coming first.
Diamond
Corundum
Topaz
Quartz
Iridium
Apatite
Bell Metal
Boric Acid
Rock Salt
Kaolin
Take Kaolin as a basis. The number opposite each substance in the
following list indicates its comparative degree of hardness in relation
to Kaolin: