Zircon
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.