Zircon is a naturally occurring mineral, known chemically as zirconium silicate (\(\text{ZrSiO}_4\)). It is considered one of the oldest materials on Earth, with crystals found dating back over 4.4 billion years. Its longevity in geological settings speaks to its resilience against heat, pressure, and chemical weathering. For consumers, the mineral’s hardness and overall durability determine its suitability for use in jewelry and various industrial applications.
Understanding Mineral Hardness
The question of how hard zircon is can be answered by consulting the Mohs Scale of Mineral Hardness, a standard system used to measure a material’s resistance to scratching. This scale is a relative ranking from 1, represented by the soft mineral talc, up to 10, which is the maximum hardness of a diamond. Zircon typically falls within a range of 6.5 to 7.5 on this scale, which places it firmly in the category of moderately hard gemstones.
This specific hardness rating has direct implications for the long-term wear of zircon, particularly when used in jewelry like rings. For example, the common mineral quartz, a primary component of household dust and sand, has a Mohs hardness of 7. Since zircon’s hardness range overlaps with and can be less than that of quartz, it is susceptible to scratching and abrasion from everyday exposure to dust. A mineral must have a hardness significantly higher than 7 to reliably resist scratching from these common environmental abrasives.
The variance in Mohs hardness (6.5 to 7.5) is largely due to a natural process called metamictization. This occurs over geological time as trace amounts of radioactive elements, such as uranium and thorium, decay within the mineral’s structure. The internal radiation gradually disrupts the crystalline lattice, reducing the mineral’s overall hardness. Zircon that has experienced extensive damage is known as “low zircon” and is less hard than “high zircon” with an intact structure.
Beyond Hardness: Zircon’s Durability Factors
While hardness measures scratch resistance, a mineral’s overall durability in practical use, such as a gemstone, also depends on its tenacity and cleavage properties. Tenacity describes a material’s resistance to breaking, chipping, or fracturing when subjected to impact or force. Zircon is generally described as having low to moderate toughness, which means it can be brittle and is vulnerable to damage from a sharp blow.
This relative brittleness is compounded by zircon’s cleavage, which is the tendency of a crystal to split smoothly along specific planes of weakness. Zircon has imperfect or indistinct cleavage, meaning it can break along these defined planes if struck at the right angle. This characteristic makes the mineral susceptible to chipping, particularly along the sharp edges of a faceted gemstone. Jewelers must protect vulnerable edges when cutting and setting zircon.
The combination of moderate hardness and chipping susceptibility means zircon requires more careful handling than gemstones with higher durability, such as corundum (sapphire and ruby). Corundum has a Mohs hardness of 9, making it significantly more scratch-resistant, and it also possesses a much higher tenacity. This difference in properties translates to a substantial difference in suitability for daily wear.
Zircon Versus Cubic Zirconia
One common source of confusion is the distinction between natural zircon and the synthetic material known as cubic zirconia (CZ). Despite the similar names, they are fundamentally different materials with separate origins and chemical compositions. Natural zircon is zirconium silicate (\(\text{ZrSiO}_4\)) found in the Earth’s crust, whereas cubic zirconia is a synthetic compound, zirconium dioxide (\(\text{ZrO}_2\)), created in a laboratory.
Their difference in hardness is a major point of confusion, as the synthetic material is actually harder than the natural mineral. Cubic zirconia possesses a Mohs hardness of 8 to 8.5, which is notably higher than the 6.5 to 7.5 range of natural zircon. This greater hardness allows cubic zirconia to resist scratching better than its natural namesake.
Cubic zirconia was developed as an inexpensive simulant for diamonds and remains a popular option for costume jewelry. In contrast, natural zircon is a genuine gemstone, valued for its high light dispersion, which gives it a brilliance and “fire” that rivals diamonds. Although cubic zirconia is harder, it is chemically distinct from natural zircon.