Species and chemistry
Zircon is zirconium silicate (ZrSiO₄). Mohs hardness is 6.5 to 7.5; specific gravity ranges from 3.93 to 4.73 depending on metamictization. Zircon is uniaxial and positive.
The species occurs in three structural states reflecting progressive radiation damage from radioactive trace elements (uranium and thorium):
- High zircon — fully crystalline, stable structure.
- Intermediate zircon — partially metamict; lower density and birefringence than high zircon.
- Low zircon — fully metamict; isotropic and amorphous in extreme cases.
Sources
- Cambodia — Ratanakiri province: source of the saturated blue heated zircon prevalent in the trade.
- Sri Lanka — Ratnapura: historic source of green and brown zircon.
- Tanzania — yellow and red zircon.
- Myanmar — Mogok: high-quality zircon in a range of colors.
- Australia — old workings in New South Wales and Western Australia.
Treatments
- Heat treatment of brown zircon to produce the saturated blue color of the trade is standard. Treatment is not always disclosed; some heated material reverts toward brown over time on prolonged ultraviolet exposure.
The "cubic zirconia" simulant — a synthetic zirconium oxide — is unrelated to natural zircon but shares an element of the chemical name and is occasionally conflated in retail listings.
Lapidary considerations
High zircon has high refractive index (1.92 – 1.98) and high dispersion (0.039), producing strong fire in the cut stone. The species responds to designs that emphasize dispersion, and brilliant cuts derived from diamond designs perform well.
Zircon's birefringence is sufficient to produce visible doubling at the back facet edges; orientation along the optic axis minimizes the effect. The species is comparatively brittle and chips at facet edges; care during dopping and cutting is required.
Final polish is conventionally performed at 50,000 mesh diamond paste; cerium oxide can produce orange-peel surface effects on some material.