Bort (also spelled boart) is the collective term for diamond material that is unsuitable for use as a gemstone, owing to deficiencies in clarity, colour, crystal form, or structural integrity. It encompasses a broad range of material: heavily included single crystals, irregularly formed fragments, and — most distinctively — polycrystalline diamond aggregates in which randomly oriented microcrystals are intergrown without a coherent cleavage plane. Rather than being discarded, bort is crushed into abrasive powder or grit and put to work in cutting, grinding, drilling, and lapping tools, exploiting the single property that makes diamond extraordinary regardless of its appearance: a Mohs hardness of 10, the highest of any natural material.

Definition and Classification

Within the diamond trade and gemmological literature, the term bort covers several distinct categories of sub-gem material:

• Polycrystalline bort: Intergrown masses of minute diamond crystals with no preferred orientation. Because cleavage planes run in multiple conflicting directions, polycrystalline bort is actually tougher than a single-crystal diamond of equivalent size — it resists fracture along any single plane. This variety is the most prized industrially.
• Cleavage fragments: Pieces broken from larger crystals during mining or processing, lacking the size or shape to be fashioned as gems.
• Off-colour or heavily included crystals: Single-crystal diamonds whose colour (typically dark grey, brown, or black due to graphite or other inclusions) or whose density of internal flaws renders them commercially worthless as gemstones.
• Crushing bort: Material specifically selected or processed for reduction into abrasive grit, the finest grades of which approach the particle sizes used in diamond-polishing compounds.

The boundary between bort and low-grade gem rough is not always sharply defined; the distinction is ultimately commercial rather than strictly mineralogical. A crystal rejected as bort by one cutter may, under different market conditions or with different fashioning technology, yield a small but saleable gem.

Etymology and Historical Use

The word bort is of uncertain but ancient origin. It appears in early modern Dutch and German diamond-trade vocabulary, and some authorities connect it to a Middle Dutch root meaning "bastard" or "inferior," paralleling the French bâtard as applied to off-quality goods. The spelling boart was common in older English-language trade literature and remains in occasional use today.

Historically, bort was the primary — and for centuries the only — source of diamond abrasive. The gem-cutting workshops of Antwerp and, later, Amsterdam depended on bort recovered from the alluvial fields of India and, from the eighteenth century onward, Brazil. Diamond wheels and laps charged with bort powder were essential to the fashioning of gem diamonds themselves, making bort an indispensable input to the very industry that rejected it as a gem material. The Kimberley pipes of South Africa, discovered from 1869 onward, dramatically increased the supply of both gem and industrial diamond, and bort from South African production fed the expanding demand for precision grinding tools in the industrialising economies of Europe and North America.

Principal Sources

Natural bort has been recovered from virtually every significant diamond-producing region. Certain localities, however, became particularly associated with industrial-grade material:

• Brazil (Bahia state): The carbonado deposits of Bahia — a distinct and still somewhat enigmatic form of polycrystalline black diamond — supplied much of the world's industrial diamond abrasive during the nineteenth and early twentieth centuries. Carbonado differs from ordinary bort in its porous, amorphous-looking texture and its anomalous isotopic and inclusion characteristics, which have prompted ongoing scientific debate about its origin.
• Democratic Republic of Congo (formerly Zaire): The Kasai region has historically been one of the world's largest producers of diamond by volume, with a substantial proportion of its output classified as industrial bort.
• Australia (Argyle): The Argyle mine, before its closure in 2020, produced large volumes of small, off-colour, and included diamonds, a significant fraction of which was directed to industrial use.

Physical and Structural Properties

Bort shares the fundamental chemistry of gem diamond — pure carbon crystallised in the cubic system — but its physical characteristics differ in ways that are both its commercial liability and its industrial asset. The polycrystalline structure of the most valued industrial bort eliminates the directional weakness inherent in single-crystal diamond. A gem-quality octahedral diamond cleaves readily along its {111} planes; polycrystalline bort has no such preferred fracture direction and therefore withstands the impact and lateral stress of grinding and drilling applications far better than a single crystal of the same mass.

The hardness of 10 on the Mohs scale — corresponding to a Vickers hardness of roughly 70–100 GPa depending on crystal orientation and purity — means that diamond abrasive can cut or abrade virtually any other material. This property underpins its use in:

• Wire-drawing dies for metal and fibre production
• Drill bits for oil, gas, and mineral exploration
• Grinding wheels for hard ceramics and cemented carbides
• Lapping and polishing compounds for optical glass and semiconductor wafers
• Saw blades for cutting stone, concrete, and advanced composites

Synthetic Diamond and the Decline of Natural Bort

The commercial synthesis of diamond by General Electric in 1954, using high-pressure, high-temperature (HPHT) technology, transformed the industrial diamond market over the following decades. Synthetic diamond grit, manufactured to consistent particle size and controlled purity, proved superior to natural bort in many applications and could be produced in quantities that natural mining could not approach. By the late twentieth century, synthetic diamond accounted for the overwhelming majority of industrial diamond abrasive consumed globally — estimates from industry sources have placed the synthetic share above 90 per cent of total industrial diamond volume.

Natural bort consequently occupies a much reduced commercial role today. It retains niche value in certain specialist applications where the specific toughness characteristics of natural polycrystalline material — particularly carbonado — are considered advantageous, and it continues to be recovered as a by-product of gem diamond mining, though much of it is now stockpiled or sold at minimal value. The economics of natural bort are entirely subordinate to those of the gem diamond market; bort is, in effect, what is left after the gem rough has been sorted.

Gemmological Relevance

For the practising gemmologist, bort is primarily a definitional and historical reference point rather than a material encountered in the laboratory. It is, however, relevant in several contexts. The distinction between bort and low-grade gem rough bears on the valuation of parcels from alluvial or artisanal sources, where sorting criteria may be less rigorous than in large-scale commercial operations. Additionally, the study of polycrystalline diamond — including carbonado and the related material framesite — has contributed to understanding of diamond formation environments and the deep carbon cycle, topics of active gemmological and geological research.

The term also appears in historical auction and estate records, where lots of "bort" or "boart" may be listed among the raw materials held by cutting houses or lapidaries, providing documentary evidence of trade practices in earlier centuries.

Further Reading

• GIA Gems & Gemology: Diamond
• GIA Gems & Gemology: Carbonado Diamond