Zircon is a real gemstone that ranges anywhere from Brown and red to colorless and is sometimes used as a diamond substitute. It should not be confused with Cubic Zirconia Cubic Zirconia is man made and due to durability and other reasons has remained as the most popular diamond replacement since roughly 1970

"Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. Hafnium is almost always present in quantities ranging from 1 to 4%. The crystal structure of zircon is tetragonal crystal class. The natural color of zircon varies between colorless, yellow-golden, red, brown, and green. Colorless specimens that show gem quality are a popular substitute for diamond; these specimens are also known as "Matura diamond". It is not to be confused with cubic zirconia, a synthetic substance with a completely different chemical composition. The name either derives from the Arabic word zarqun, meaning vermilion, or from the Persian zargun, meaning golden-colored. These words are corrupted into "jargoon", a term applied to light-colored zircons. Yellow zircon is called hyacinth, from a word of East Indian origin; in the Middle Ages all yellow stones of East Indian origin were called hyacinth, but today this term is restricted to the yellow zircons. Zircon is regarded as the traditional birthstone for December." Source and further information: http://en.wikipedia.org/wiki/Zircon