Colorless Ceylon zircon "Matara Diamond": identifying adamantine fire and facet doubling?
How do high refractive index (1.92+) and high birefringence (0.059) separate heat-treated white zircon from synthetic cubic zirconia and natural diamond?
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3 Answers from travellers
Colorless zircon from southern gravels—historically celebrated as the "Matara Diamond"—is natural zirconium silicate (ZrSiO4) heated in charcoal kilns to bleach brown tones. While possessing an adamantine luster and high dispersion (fire) resembling diamond, it is identified under a 10x loupe by its extreme birefringence (0.059). Looking down through the table facet at pavilion facet edges reveals distinct doubling of facet junctions. Diamond and synthetic cubic zirconia belong to the isometric crystal system and are singly refractive, showing zero facet doubling.
White zircon is brittle with a hardness of 7.5; faceted stones shipped loose in parcel papers regularly display chipped, abraded facet edges (paper wear).
Specific gravity provides a conclusive bench test: zircon has a high density of 4.60-4.70, sinking rapidly in standard 3.32 specific gravity immersion liquids.
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