Zircon is a colourful gem with high refraction and fire that’s unfairly confused with cubic zirconia.

Colourless zircon is renowned for its brilliance and the captivating flashes of multi-coloured light, known as fire, that it displays. These qualities have led to centuries of confusion with diamonds, as their optical properties are strikingly similar.

Zircon is found in a wide range of colours, including yellow, green, red, reddish-brown, and blue, making it a favourite among collectors and discerning gem enthusiasts.

Zircon crystals form in various types of rock and exhibit a range of optical and physical properties. However, some zircons, particularly green ones, show significantly reduced values in these properties. This degradation is due to radioactive elements—often present as impurities—that have damaged the crystal structure over geological time.

Gemmologists classify zircons into three categories based on the extent of this structural damage:

• High (or normal) zircons: These have well-preserved crystal structures with minimal damage from radioactive elements, exhibiting the full range of typical physical and optical properties associated with zircon.

• Intermediate (or medium) zircons: These display moderate structural damage due to radiation, resulting in properties that fall between those of high and low types.

• Low zircons: These have undergone extensive damage to their crystal structure from radioactive elements. In extreme cases, the damage can make the structure nearly amorphous, meaning it lacks an orderly arrangement.

Most zircons used in jewelry are of the high type. Interestingly, some of the damage from radiation-induced crystal breakdown can be partially reversed by heating the zircon to high temperatures, which helps repair the damaged crystal structure.