What is Cubic Zirconia?

Cubic Zirconia (sometimes called simply 'CZ') is the most popular diamond simulant in the world today. It is an oxide of the metallic element zirconium, ZrO². It has a hardness of about 8.5 on the Mohs hardness scale, as opposed to diamond, which is 10, sapphire/ruby which is 9, topaz which is 8, and so on. The refractive index of CZ is 2.42. It can be made in nearly any color and can be faceted into many cuts. 

CZ occurs naturally but in pieces too small for jewelry purposes. It wasn't until the 1970s however when Soviet scientists learned how to grow this mineral in the laboratory, at which time jewellery designers first took notice of cubic zirconia. For the first few years it was on the market it was often sold for as much as $20 per carat and was only available in a few colors. 

CZ is more dense than diamond - it weighs more for its size than diamond does. Sapphire and ruby, both of which are the same gemstone, just different colors, are also more dense than diamond. CZ is about 75% heavier than diamond. As such, a CZ's size is referred to in carats usually in comparison to diamonds. CZs are more accurately measured in millimeters, referring to the width of the stone. A 6.5mm Cubic Zirconia is equal in size to a one-carat diamond and actually weighs about 1.75 carats. 

CZ isn't grown/created like any other imitation gemstone in the world, either. A high radio-frequency "skull crucible" system is used, in which the melting zirconia powder actually creates the sides of its own container during its formation. Cooling this extremely hot molten ore becomes the most crucial step in the entire process. A carefully programmed cooling procedure is required to form the flawless crystals - metal pipes in a coil-shape with water running through them are used to cool down the material, in the centre.
Cubic Zirconia is durable, very bright, can be grown in a varity of colors and when well cut it is a very convincing diamond substitute.
- Urban Dictionary