practical gemology

Zircon

green zircon

Name Zircon
Varieties Hyacinth, Starlite ,Etc..
Crystallography Tetragonal. Crystals prismatic, pyramidal; often twinned; rounded pebbles.
Crystallographic Forms
Zircon MH
Zircon 009
Zircon 024
Zircon 066
Zircon 081
Zircon 097
Zircon 133
Zircon 137
Refractive Index  Varies by the amount of radioactive damage to its crystal structure. Low (most damaged): 1.78 – 1.85; Intermediate: 1.85 – 1.93; High (least damaged): 1.92 – 2.01. See “Varieties” for more information.
Colors  Reddish-brown, yellow, gray, green, red; various other colors induced by heating.
Luster  Vitreous to adamantine; sometimes greasy.
Polish Luster Vitreous to adamantine
Fracture Luster Vitreous to subadamantine
Hardness  6 (Low), 7.5 (High)
Fracture  Conchoidal
Specific Gravity  Low 3.95-4.20, Intermediate 4.08-4.60, High 4.60-4.80
Birefringence  Varies by the amount of radioactive damage. 0 (isotropic)-0.059. See “Varieties” for more information.
Cleavage  Imperfect
Dispersion  0.039 for all zircon types.
Heat Sensitivity None
Luminescence  See “Identifying Characteristics.”
Luminescence Present Yes
Luminescence Type Fluorescent, Phosphorescent, UV-Long, UV-Short, X-ray Colors
Enhancements Virtually all blue zircon is heat treated.
Typical Treatments Heat Treatment
Special Care Instructions Facet edges wear off. Use protective settings for ring use.
Transparency  Transparent to opaque.
Absorption Spectrum  See “Identifying Characteristics.”
Phenomena  Chatoyancy, rare
Birthstone  December
Formula ZrSiO­4 + Fe, U, Th ( Zirconium Silicate )
Pleochroism  Distinct in blue stones: deep sky blue/colorless to yellowish gray. Red: red/clove brown. Brown: reddish-brown/yellowish brown.
Optics  Uniaxial (+).
Optic Sign Uniaxial +
Etymology From the Arabic zargun, from the Persian zar for “gold” plus gun for “color.” The name is ancient.
Occurrence In igneous rocks worldwide, especially granites. Also found as alluvial material.
Inclusions Angular zoning and streaks are sometimes seen in the low type. Some silk is seen occasionally, as well as tension cracks and epigenetic cracks stained with iron oxides. Metamict pieces may have bright fissures known as angles.