Tourmaline ranks 7 to 7.5 on the Mohs hardness scale.
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.
Tourmaline ranks 7 to 7.5 on the Mohs scale. Its toughness is considered Fair.
Tourmaline is generally stable to light and isn’t affected by exposure to chemicals, but heat can damage a tourmaline. High heat can alter the color, and sudden temperature change (thermal shock) can cause fracturing.
Some tourmalines might have been treated to improve their color. The two most important tourmaline treatments are heating and irradiation.
Changes resulting from heat treatment are stable and undetectable. Gems with abundant liquid inclusions can’t withstand heat treatment.
Color changes due to irradiation can fade with exposure to heat or bright light.
Warm, soapy water is the best method for cleaning tourmaline. The use of ultrasonic and steam cleaners is not recommended.