Gemstone and Mineral Durability is measured on the “Mohs Hardness scale” – which refers to a stone’s resistance to scratching. 1 is least resistant and 10 is the most – the only gemstone on the hardness scale that is a 10 is diamond. However, a gem that has a high number on the hardness scale may still be delicate in nature.
Toughness refers to a gemstone’s resistance to cracking and breaking. For instance, emerald is fairly high on the hardness scale at 7 1/2-8, but it is not a tough stone – and needs to be treated and maintained with care.
You can learn more about the Mohs Scale at the Gemological Institute of America (GIA).