The hardest mineral on the scale, diamond, is given a hardness rating of 10. Since obsidian is rated at 5-6 on Moh's Hardness Scale, it is not stronger than a diamond; it's softer than a diamond. These transformed and deformed products of heat and pressure are called metamorphic rocks. Metamorphic rocks do not form at the Earth's surface as the heat and/or pressures required for metamorphism are found kilometers deep within the lithosphere.