Cubic zirconia (CZ), the cubic crystalline form of zirconium dioxide (ZrO2), is a mineral that is widely synthesized for use as a diamond simulant. The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirconium silicate (ZrSiO4).

Because of its low cost, durability, and close visual likeness to diamond, cubic zirconia has remained the most gemologically and economically important diamond simulant since 1976. Its main competition as a synthetic gemstone is the more recently cultivated material.

Cubic zirconia is relatively hard, at about 8.5 on the Mohs scale— much harder than most natural gems. Its refractive index is high at 2.15–2.18 (B-G interval) and its luster is sub adamantine. Its dispersion is very high at 0.058–0.066, exceeding that of diamond (0.044).