Devised by the German mineralogist Friedrich Mohs, the scale is a system to measure the relative hardness and durability of gemstones.
Moh's selected ten minerals and gave each of them a number according to their 'scratching order'. Put simply each gemstone will scratch the ones below it, and will be scratched by the ones above it in the scale.
The softest was talc, which scored one, while the hardest was diamond, which therefore got a ten. Everything else came somewhere in the middle. Rubies and sapphires were found to be extremely scratch resistant scoring a 9, and are therefore good for rings; emeralds at 8 are a little softer, while amber at 2.5 will be easily scratched and is usually better worn as earrings, or as a necklace.
6 Orthoclase Feldspar
9 Corundum (sapphire)