Zircon
December Birthstone
Zircon is the birthstone for those born in the month of December.
What is Zircon?
Zircon is a popular gemstone that has been used for nearly 2000 years. Today it serves as a modern birthstone for the month of December.
Zircon occurs in a wide range of colors and has a brightness and fire that rivals those of diamond. Colorless zircon is sometimes used as a lower-cost alternative for diamond. Zircon should not be confused with cubic zirconia, which is a man-made material.
Occurrence?
Zircon is present in most soils and clastic sediments. Zircon-rich sediments are mined and the recovered zircon is used to produce zirconium metal and zirconium dioxide.
Its very high dispersion and refractive index give it a brilliance and fire that rival those of diamond. For this reason, colourless faceted zircon has been used as both a popular substitute for diamond.

Source: Geology.Com