Zircon

White Zircon: Zirconium Silicate Zr[Si04]; crystal system: Trigonal; name derived from the Persian Zargun meaning golden; Relative Density: 4.6-4.7; Hardness: 7.5; Cleavage: Poor; Tenacity: Brittle; Refractive Index: 1.923-2.015; Birefringence 0.058; Dispersion:Strong; Pleochroism: Weak.

This is a good simulant for diamond, it is hard has a high refractive index, and good dispersion. It is visually lacking in life by comparison to diamond, but has more than both Topaz and Sapphire. However it is easy to separate with an eye glass, having a large double refraction. Viewing a back facet through the table with a lens will reveal doubling of the edges. Zircon has a distinctive appearance which is easily recognisable once noticed.

Original document: http://www.jewellerycatalogue.co.uk/diamonds/simulants_info.php