Zircon is a silicate mineral. It is found in many different places in almost all kinds of rock. The chemical it is made up of is called "zirconium silicate": ZrSiO4. Clear zircon may look like diamond and has been used as a cheaper replacement for diamond. It is not the same as cubic zirconia, which is man-made zirconium dioxide. In the United States, zircon is considered to be the birth stone for the month of December.