Cubic Zirconia also known as CZ is a diamond stimulant. A diamond stimulant is something that looks like a diamond but is not a diamond. Do not confuse a stimulant with a synthetic diamond which is actually a diamond whereas a stimulant is not a diamond.
CZ is chemically known as zirconium dioxide (ZrO2). It is manufactured fairly cheaply and looks very similar to a diamond so it is used in almost all cheap jewellery.
Commercial manufacturing of CZ started in 1977. One of the techniques to manufacture CZ uses a technique known as skull crucible where the material (in our case zirconium oxide) forms its own crucible. This is done due to the very high melting point of CZ.