The stone hardness scale is commonly known as the Mohs scale of mineral hardness. The scales ranges from 1-10, where 1 is the softest and 10 is the hardest. It ranks minerals based on their scratch resistance and not for their fracture resistance.
Rankings:
1 - Talc
2 - Gypsum
3 - Calcite
4 - Fluorite
5 - Apatite
6 - Orthoclase (Feldspar)
7 - Quartz
8 - Topaz
9 - Corundum (Sapphires and Rubies)
10 - Diamond
This helps you asses the durability of each stone making it so that you know which gems are suitable for everyday wear (harder stones are generally better for daily wear). It also informs you about their care and maintenance, as softer stones may need to be handled more carefully compared to harder counterparts.