Cubic zirconia

Cubic Zirconia, CZ for short, is a synthetic gem composed of zirconium oxide, ZrO₂. It has a cubic crystal system, a refractive index of 2.15-2.18, dispersion of 0.065 (high), specific gravity of 5.6-6, hardness of 8.5, an adamantine lustre, yellow-green to yellow luminescence in short wave ultraviolet light and whitish luminescence in long wave ultraviolet.

Zirconium is quite refractory, thus a special mechanism called a skull crucible is used to produce cubic zirconium.