Zircon is said to be the oldest gem on earth and is far older than even the most ancient Diamond. The colourless variety is the best known, looking more like a Diamond than any other natural stone due to its luminosity and dispersion.
One of the reasons the gem has such amazing brilliance is that it is doubly refractive. As light enters it splits in two and the facets on the pavilion act like a wall of mirrors.
Zircon from the Ratanakiri region of Cambodia can display magnificent Paraiba-like blue-greens.