colorless, very strong blue to green-blue, yellow, blue-green, yellowish green, yellow-green, brown, orangy yellow to reddish orange, dark brownish red and sometimes purple, gray to bluish gray and brownish gray
high zircon, medium zircon, low zircon or metamict, hyacinth, jacinth, jargoon or jargon, beccarite, melichrysos, sparklite, starlite, stremlite
Zircon exhibits strong doubling, easily observed using a 10x loupe as a doubling of back facet edges. The refractive index of zircon is over the limit of the standard gemological refractometer. Spectrum should show a distinct line at 653.5nm, but possibly up to 40 lines. Low type zircon's spectra might have fuzzy lines and not well defined. OTL RI reading, eye visable doubling and spectrum. Some low type zircon appears to be SR and amorphous.
Zircon affected by natural radiation may have altered crystal lattice structure that can affect some of its measurable properties. Crystals are prismatic in a tetragonal form, often with bipyramidal terminations, with square or rectangular cross-sections. Always look for doubling in 3 directions.Misnomers: Colorless zircon- Matara diamond, Blue zircon- Siam aquamarine.High and medium zircon: transparent colorless, blue, yellow to yellow-green, brownish green, orange to brown, brownish to orangy red.Low or metamict zircon: transparent brownish to yellowish green with a cloudy texture; more rarely brown to orange.Hyacinth or jacinth: yellow-red, orange, red-brown to brown.Jargoon or jargon: light yellow to colorless stones that do not show the red color of hyacinth. Traditionally they are stones from Sri Lanka.Beccarite: green zircon that displays optical properties. Melichrysos: straw yellow.Starlite or stremlite: blue heat treated zircon.Sparklite: colorless zirconHigh type RI: 1.925 to 1.984(+/-0.040); Medium type RI: 1.875 to 1.905(+/-0.030); Low type RI: 1.810 to 1.815 rarely under 1.800Low type has low birefringence and high type has high birefringence. Blue is created by heating reddish brown stones.The green metamict zircon might be slightly radioactive.