Mohs scale of hardness

Devised by the German mineralogist Friedrich Mohs (1773-1839), this scale is a practical method of comparing the different levels of hardness of different minerals and gems. It is measured in terms of the ability of a material to resist scratching by those above it in the scale. It is a table of 10 different minerals, arranged in order from Talc the softest at number 1 to Diamond the hardest at number 10.
It should be noted that the levels of hardness are relative and the difference in actual hardness is greater between numbers 9 and 10 than between 1 and 9!

Fact: As a comparison with some more usually encountered materials in terms of hardness:

Graphite (pencil lead) – 1.5 Mohs

A fingernail – 2.5 Mohs

Window plate glass – 5.5 Mohs

Steel file – 6.5 Mohs