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.
Zircon is rated 6 to 7.5 on the Mohs scale. It has fair to good toughness. While zircon is not as hard or tough as a stone like sapphire, it can still be worn in all types of jewelry as long as proper care is given to avoid rough wearing or hard blows. Also, zircon can be brittle, so it should be worn and stored in a manner that avoids hard rubbing against other materials.
Zircon is generally stable when exposed to light, but some heat-treated stones might revert back to their original colors (usually light brown) after prolonged exposure to bright light. Heat can alter the color of some zircon. Zircon is stable when exposed to chemicals.
Warm, soapy water is recommended for cleaning zircon. Ultrasonic and steam cleaners are not recommended for cleaning this gem.
Zircon is commonly heat-treated to produce blue and colorless varieties. As with any transparent gemstone, zircon can be fracture-filled to improve apparent clarity, but this treatment is rarely applied to this gem.