Hardness

Hardness is the ability of a crystal to withstand surface scratching and is measured by the Hardness scale invented by Freidrich Mohs in 1822.

The scale relates to the strength of the structure in relation to the chemical bonds that cohere it; small, tightly packed atoms and strong bonds make for a hard stone.

The scale progresses from 1, the softest, to 10, the hardest, although the progression is not uniform. Diamond, for instance, the hardest known mineral, is a perfect 10 while Sapphire is 9 and Topaz is 8, but Sapphire and Topaz are much closer to each other in hardness than Sapphire is to Diamond, which is immensely hard.

Gemstones need to be at a minimum of 7 on teh scale to withstand constant wear, and soft minerals such as talc are often used as lubricants. Each mineral will scratch itself or something lower on the scale, but will not scratch anything higher.


Simple Equivalents Are:

2 - Marks with a fingernail
3 - Marks with a copper coin
4 - Marks easily with a knife blade
5 - Marks with a knife blade with difficulty
6 - Marks with a steel file
7 - Scratches window glass