Cubic Zirconia (CZ) is a synthetic material composed of crystalline zirconium dioxide, widely used as a diamond simulant in jewelry. It is grown in a lab setting to achieve a clear and colorless appearance. Cubic zirconia does not float in water; it sinks immediately. This behavior is a direct consequence of its physical makeup, specifically the amount of mass packed into its volume, a property known as density.
The Role of Density: Why Cubic Zirconia Sinks
The act of whether an object floats or sinks in water is governed by the principle of density, which is mass per unit volume. For comparison purposes, scientists use specific gravity (SG), a ratio comparing a material’s density to the density of water. Water has an SG of 1.0; any substance with an SG greater than 1.0 will sink, while anything less than 1.0 will float.
Cubic zirconia possesses a high specific gravity, typically falling within the range of 5.6 to 6.0. This means CZ is approximately five and a half to six times denser than an equal volume of water. When a CZ stone is placed into water, the downward gravitational force acting on the material far outweighs the upward buoyant force exerted by the displaced water.
Because the CZ stone is much heavier than the water surrounding it, it cannot be supported and drops to the bottom of the container. This high density is an inherent feature of its tightly packed crystal structure, which contains the relatively heavy element zirconium. The sinking action demonstrates the stone’s high specific gravity.
Comparing CZ to Other Gemstones and the Flotation Test Misconception
People often ask if CZ floats because they want to use the water test as a simple identification method. However, attempting to distinguish between cubic zirconia and diamond using only water is ineffective because both materials are significantly denser than the liquid. Diamond has a specific gravity of about 3.52, which is still over three and a half times denser than water.
Since both CZ (SG 5.6–6.0) and diamond (SG 3.52) have specific gravity values well above 1.0, both will sink swiftly to the bottom. The difference in their densities, while substantial to a gemologist, is not large enough to change the outcome of the float test. Even common simulants, such as soda-lime glass, have a specific gravity around 2.5, ensuring they also sink.
The flotation test is only useful for distinguishing materials with an SG near 1.0, such as amber, or materials less dense than water, like certain plastics. Therefore, the water test provides no reliable information for identifying or separating CZ from a diamond or other common gemstone simulants. Accurate measurement of the density difference between stones requires specialized hydrostatic weighing equipment, not a simple visual observation in a glass of water.