The Mohs Scale is commonly used to rate gemstone hardness.
It’s very useful to know the hardness of the particular gemstone you’re working with so that you can design a mounting or piece of jewelry appropriate for it, resulting in a quality, durable piece of jewelry. Softer stones, for instance, are not usually so great for rings, but make great shorter-length necklaces. Don’t let your customers find this out for you! Another example is that you can apply a pumice wheel, which is a 5.5 on the Mohs scale, to your bezel or prongs during the finishing process without worry about damage to your stone as long as it’s a 6 or greater on the Mohs scale.