The Mohs Scale rates minerals in terms of hardness on a scale from 1 to 10, where 10 is the hardest mineral (Diamond) and 1 is the softest and the most prone to scratches (Talc). The minerals are measured by their ability to scratch other minerals. Most black gemstones suitable for jewelry measure 5 or above on Mohs scale. Softer gems that are uniquely appealing make great jewelry pieces as well (such as pearls).