Kalimantan is the Indonesian portion of the island of Borneo, occupying roughly the southern three-quarters of the world's third-largest island. Within the gem trade the name signals two things in particular: a centuries-old artisanal alluvial diamond field, and a secondary source of corundum, zircon and chrysoberyl recovered from the same drainage systems.

The diamond field

Diamond mining in Kalimantan is concentrated in the Martapura-Cempaka district of South Kalimantan province (Kalimantan Selatan), within the watershed of the Riam Kanan and its tributaries near Banjarmasin. The deposits are alluvial, with the primary kimberlitic source rocks not yet identified despite decades of exploration; the working assumption is that the diamonds derive from cratonic basement to the south or east and have been redistributed through Tertiary sediments. Production is dominated by small-scale artisanal pits worked by local cooperatives using sluices and hand-jigs, with periodic recoveries of larger stones drawing international attention — the most famous historical example being the Trisakti, a 167-carat rough recovered in 1965 and named after the Indonesian state principles.

Kalimantan diamonds are typically small, with most production under 1 carat and gem-quality material concentrated below 0.5 carats, although individual stones in the 5- to 50-carat range emerge regularly enough to support a steady flow of significant rough into the Martapura cutting market. Quality leans towards near-colourless to faint yellow, often with strong fluorescence; the cut goods historically supplied the Indonesian and broader Southeast Asian regional trade rather than the international wholesale market.

Coloured stones and other gems

The same drainages yield zircon (sometimes heat-treated locally to blue), spinel, almandine garnet, sapphire and ruby in modest quantities. Borneo is also a noted source of agate and chrysoprase from western Kalimantan. The corundum, while genuine, is generally not of fine gem quality and competes poorly with material from Madagascar, Sri Lanka and East Africa; it is most often consumed in the regional Indonesian trade.

Cutting centres and trade structure

Martapura functions as the cutting and trading hub for the South Kalimantan field. The town's pasar intan (diamond market) trades parcels of local rough and finished goods, with the cutting style historically biased toward older proportions optimised for weight retention rather than international brilliance grades. Indonesian regulatory frameworks treat artisanal diamond mining under a permit (IPR) regime that has shifted several times since 2009; export of polished goods is legal but the formal share of production passing through licensed channels has fluctuated.

Trade significance

For the international gem trade Kalimantan is a niche origin. It is referenced for historical interest, for occasional notable recoveries, and as a source location on disclosure where stones can be tracked back to the Martapura cooperatives. It is not a meaningful supply source for the contemporary global rough market, and Kalimantan-origin material rarely reaches the major Antwerp, Mumbai or Tel Aviv polishing centres in identifiable quantity. Stones with documented Borneo provenance can carry collector interest, particularly for natural-coloured Indonesian alluvial diamonds, but they are not graded with origin premium in the way Kashmir sapphires or Mogok rubies are.