Marlborough chrysoprase is the trade name for the apple-green, nickel-coloured chalcedony produced from the Marlborough mining district of central Queensland, Australia. It is widely regarded as the international quality benchmark for chrysoprase, against which material from competing sources such as Poland, Tanzania, Western Australia and Brazil is compared.

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Mineralogy

Chrysoprase is a cryptocrystalline quartz, structurally a chalcedony, in which the green colour is produced by minute inclusions of nickel-bearing minerals dispersed through the silica matrix. Refractive index is approximately 1.53 to 1.54, specific gravity about 2.58 to 2.64, and hardness 6.5 to 7. It appears singly refractive in standard gem testing, and may show a very weak nickel-related absorption band near 650 nm under optimal viewing conditions.

The Marlborough deposits are hosted in a deeply weathered serpentinite, with the chrysoprase forming as veinlets, nodules and matrix replacements within a lateritic saprolite. Nickel from the ultramafic parent rock has been mobilised by tropical weathering and reprecipitated into silica gel, which subsequently dehydrated and crystallised as the cryptocrystalline quartz the trade now sells.

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Quality factors

The dominant factor in Marlborough chrysoprase is colour. The most prized goods are a pure apple green, leaning very slightly toward bluish rather than yellowish, with a saturation that reads as luminous when set. Lower grades are paler, greyer or more yellowish, and are consigned to carving and inlay rather than calibrated cabochons.

Translucency is the second factor: the best stones are nearly transparent, while commercial goods are translucent to slightly translucent. Surface texture is typically smooth and waxy in polish, and the cryptocrystalline structure means that a vitreous polish requires careful lapping. Inclusions of brown limonite or black manganese oxide can be tolerated in lower grades but are downgrading on top goods.

The material is essentially never enhanced, although stabilisation with polymer impregnation occurs occasionally on porous low-grade material and should be disclosed. The colour is light-stable in the sense that prolonged exposure to high temperature can cause some loss of saturation, and stones intended for daily wear should be protected from prolonged direct sunlight in dry climates.

Trade

Marlborough chrysoprase is sold as calibrated cabochons, freeforms, and beads, with the highest grades cut into doublets or set into one-of-a-kind designer pieces. There is no laboratory origin determination for the material, and trade attribution to Marlborough is supported by parcel documentation from the original mining operator or from a credible Australian dealer.

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