Kauri gum is the hardened resin of Agathis australis, the kauri tree of New Zealand's North Island. Although it is often grouped commercially with amber, the trade and the gemmological literature classify it more cautiously: most kauri gum recovered for ornamental use is sub-fossil rather than truly fossilised, with ages typically measured in thousands rather than millions of years. It is best understood as a high-quality copal that has, in some deposits, advanced toward an amber-like state.

Origin and recovery

The material formed where ancient kauri forests once stood across what are now the gum-bearing peatlands of Northland and the Coromandel. Resin exuded from living trees, fell to the ground with branches and bark, and was buried in swampy soils that preserved it from oxidation. Settlers and Dalmatian gum-diggers worked these fields intensively from the 1840s through the early twentieth century, probing the ground with steel spears and digging out lumps that ranged from pea-sized fragments to masses of several kilograms. The Te Papa Tongarewa national collection in Wellington holds documentary material on this history, as does the Kauri Museum at Matakohe.

Composition and gemmological character

Kauri gum is a complex mixture of diterpenoid resin acids, dominated by communic-acid derivatives, with a refractive index near 1.54 and a specific gravity around 1.04 to 1.06. It floats in saturated salt water, a useful field test it shares with amber. Hardness sits at roughly 1.5 to 2.5 on the Mohs scale, softer than most ambers, and the surface is more prone to crazing under heat or solvents. Colour ranges from pale straw and honey through reddish-brown to near-black, with the most prized pieces showing translucent golden bodies and occasional plant or insect inclusions.

Distinguishing from amber

The distinction matters in trade. True amber, by the working definition used by laboratories such as those reviewed in Gems & Gemology, has undergone polymerisation and loss of volatiles over geological time and resists most organic solvents. Kauri gum, being younger and less polymerised, softens and dissolves in ether or chloroform within seconds, where Baltic amber resists. Long-wave ultraviolet response is bluish-white and often more vivid than that of fossilised ambers. Sellers describing kauri gum as amber without qualification mislead the buyer; reputable dealers label it as kauri gum or, occasionally, as New Zealand copal.

Use in jewellery and carving

Historically the bulk of harvested gum went to industrial uses, principally varnish manufacture, where it gave a hard, pale film. Ornamental use was always secondary, although Maori carvers and later Pakeha lapidaries produced beads, pendants, and small carvings. Pieces with insect or plant inclusions command a premium on the collector market. The softness limits everyday wear, and modern jewellers generally set kauri gum in protected mountings or reserve it for brooches, pendants, and earrings rather than rings.

Status today

Commercial digging effectively ended in the mid-twentieth century, and the remaining supply comes from old stockpiles, occasional finds during land works, and small-scale recovery on private property. The living kauri is itself protected, and resin is not collected from standing trees. Buyers should expect modest prices for plain pieces and significantly higher figures for clear, large, or inclusion-bearing specimens with documented provenance.