Marlborough is a small town and surrounding mining district in central Queensland, Australia, located inland from the coastal city of Rockhampton. It is internationally significant in the gem trade as the principal commercial source of fine chrysoprase, the apple-green chalcedony that has dominated the cabochon and carving markets for that material for the better part of half a century.

Geological setting

The Marlborough chrysoprase deposits sit within a serpentinite belt of Permian age that has been weathered to form a thick lateritic cap. The chrysoprase forms in fissures, veins and nodules within this saprolite, with its colour derived from nickel that was leached from the parent ultramafic rocks and concentrated in the silica gel that subsequently crystallised as cryptocrystalline quartz. The ICA and successive editions of GIA's Gems & Gemology coverage of Australian gem materials note that Marlborough material has been the international benchmark for top-grade chrysoprase since the 1960s.

Mining in the district has historically been conducted by small operators working open-pit and shallow strip workings. Production is irregular, dependent on the encounter rate of pockets within the saprolite, and parcels are sold either to Australian cutting houses or, more often, exported in rough form to cutters in Asia and Europe.

Trade and product

For the trade, Marlborough is the byword for fine chrysoprase: a saturated, slightly bluish apple green, translucent to slightly translucent, free of grey or yellow modifying tones, and capable of polishing to a vitreous lustre. Lower grades are sold by weight as carving rough or for inlays, while top grades are calibrated cabochons supplied to bench jewellers and major brands. There is no laboratory origin determination for Marlborough chrysoprase that distinguishes it from chrysoprase of other Australian or international sources, so origin attribution rests on parcel-level trade documentation rather than on a certificate.

The town and district remain the most-cited entry under "chrysoprase" in modern gem-locality references, and any discussion of fine chrysoprase in the international trade will, sooner or later, return to Marlborough.