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.
Alexandrite is relatively hard—8.5 on the Mohs scale. It has excellent toughness and no cleavage, which is a tendency to break when struck. This makes it a good choice for rings and other mountings subject to daily wear.
Alexandrite is stable under normal wearing conditions, which means it’s resistant to the effects of heat, light, and common chemicals.
Warm, soapy water is always safe for cleaning alexandrite. Ultrasonic and steam cleaners are usually safe.
Treatment and durability considerations
Alexandrites are usually not treated, although they might have fractures. Fracture-filled gemstones should only be cleaned with warm, soapy water.