Scratch Hardness



Scratch Hardness

The Viennese mineralogist Friedrich Mohs (1773-1839) introduced the term scratch hardness. He defined the scratch hardness as the resistance of a mineral when scratched with a pointed testing object.

Gemstones Hardness Scale

Mohs set up a comparison scale using ten minerals of different degrees of hardness, which is still widely in use. Number 1 is the softest, number 10 is the hardest. Each mineral in the series scratches the previous one with lesser hardness and is scratched by the one which follows after.
Gemstones are often tested by using the Mohs’ hardness scale to determine just how hard they are. The harder minerals are more durable in that they do not scratch easily and will hold up better in jewelry. Talc is the softest mineral with a hardness of 1 and can be easily scratched with a fingernail. The gemstones with a rating of 7 or over are relatively hard. Quartz gemstones (citrine, amethyst, etc.) range in the 7's, topaz rates 8, and corundum (sapphires and rubies) are a 9 on the Mohs' hardness scale. Diamond registers a 10 and is the hardest known naturally occurring material on earth, more than ten times the hardness of corundum at 9. There is more of a spread between the gems and minerals found between 2 and 3 and between 5 and 6, however corundum is only about 10 per cent harder than topaz.

The hardness is relative, but it is, nevertheless, a useful identification tool. Hardness is almost never used as a separation test with gemstones since it is considered a destructive test and other nondestructive tests exist to enable separation and identification.