Hardness and toughness
Gem and mineral hardness is measured on the Mohs scale. The numbers are based on the relative ease or difficulty with which one mineral can be scratched by another. But the Mohs scale is deceptive. The steps between the minerals are not evenly spaced. For example, diamond is only one number away, but it’s many times harder than gems in the corundum family.
Moonstone falls between 6 and 6.5 on the Mohs scale. It also has poor toughness.
Moonstone has two cleavage directions, a trait it shares with the other members of the feldspar family of minerals. Cleavage is a direction of atomic weakness that makes a crystal more prone to breaking. Cleavage can affect durability, so manufacturers usually set moonstone into pendants, earrings, and pins rather than rings, which are more likely to receive knocks and bumps.
High heat or sudden temperature change can cause breaks in moonstone. It is stable to light but is harmed by exposure to hydrofluoric acid.
Warm soapy water is the only recommended substance for cleaning moonstones. Ultrasonic and steam cleaners are never recommended.