Cubic Zirconia (CZ) is a synthetic mineral/stone made of zirconium dioxide, most commonly used in jewelry because they look so enchanting. Due to its optical properties it's also been used for windows, lenses, prisms, filters and laser elements.
Cubic zirconia have actually been found in nature in small amounts, but the majority used in jewelry are man-made in a lab. It doesn't have the same carbon structure as diamonds. Synthetic diamonds do – and are also made in labs, often called "lab-grown" diamonds.
When it comes to hardness cubic zirconia is surprisingly hard for a synthetic stone, being rated 8/10 on the Mohs hardness scale. Diamonds are rated a clear 10/10 on this scale.
Cubic zirconia is a much more affordable alternative to diamonds. They come in many colors, but white/"transparent" stones are the most powerful when in comes to playing with color because of light dispersion. This light dispersion is actually stronger than in a diamond.
Because of its low cost, durability, and close visual likeness to diamond, cubic zirconia has remained the most important competitor to diamonds since 1976. Its main competitor as a synthetic gemstone is a more recently "invented" material, synthetic moissanite.