A bruting diamond is a diamond used as an abrasive tool to grind and round another diamond during the process known as bruting — the first shaping stage in the manufacture of a round brilliant or other circular-outline cut. Because diamond is the hardest natural substance known, rated 10 on the Mohs scale, only diamond is capable of abrading diamond efficiently; no substitute mineral approaches the hardness required. Bruting diamonds are therefore an indispensable industrial consumable within the cutting and polishing trade, typically sourced from lower-quality rough that is unsuitable for faceting on account of heavy inclusions, poor colour, or unfavourable crystal geometry.

The Bruting Process

Bruting — also called girdling or rounding — is the stage at which a sawn or cleaved diamond preform is given a circular cross-section before the cutter proceeds to faceting. In the traditional manual method, two diamonds are mounted in opposing dops (small metal holders) on a lathe-like spindle. One stone rotates at speed while the other, the bruting diamond, is held against it under controlled pressure. The two stones abrade each other simultaneously, each removing material from the other's girdle zone. The result is a cylindrical outline — the girdle — that serves as the geometric foundation for all subsequent faceting work.

The abrasion is purely mechanical: diamond scratches diamond along crystallographic directions of relative weakness, generating the fine diamond powder (bort) that falls away during the operation. This powder is itself a valuable by-product, collected and reused as polishing abrasive on lapping wheels.

Material Selection for Bruting Diamonds

Because both stones in the bruting pair are consumed — each losing material to the other — the economics of the process demand that the bruting diamond be of the lowest acceptable grade. Cutters and manufacturers typically select:

• Heavily included rough of type that would yield a commercially unattractive faceted stone.
• Off-colour material in the brown or grey range, where the colour penalty in the faceted market is severe.
• Irregularly shaped crystals, macles, or flattened naats that do not lend themselves to standard sawing and faceting yields.
• Small-diameter rough whose faceted yield would be uneconomically low relative to cutting labour costs.

The hardness of diamond is, of course, identical regardless of clarity or colour grade; a heavily included stone abrades just as effectively as a flawless one. This makes the repurposing of otherwise unmarketable rough as bruting material a logical and long-established trade practice.

Historical Development

The technique of using one diamond to shape another is ancient in principle, though the organised industrial application of the bruting lathe developed in the major cutting centres of Antwerp and Amsterdam from the fifteenth century onward, as the demand for symmetrical round stones grew alongside advances in faceting. Early bruting was entirely manual, requiring skilled craftsmen to judge pressure and angle by touch and experience. By the late nineteenth century, mechanised lathes with adjustable spindle speeds had become standard in the Antwerp and later the New York and Tel Aviv cutting industries.

The introduction of the round brilliant cut as a mathematical ideal — formalised by Marcel Tolkowsky in his 1919 treatise Diamond Design — placed new demands on the precision of the bruted girdle, since the proportions of the finished brilliant depend on a consistently circular and well-centred outline. This elevated the importance of controlled, repeatable bruting over purely manual technique.

Modern Automated Bruting

Contemporary large-scale diamond manufacturing facilities, particularly those operating in Surat, India — which processes the vast majority of the world's cut diamonds by volume — employ computer-numerically-controlled (CNC) bruting machines. These systems hold both the work stone and the bruting diamond in motorised dops, regulate rotational speed and lateral feed automatically, and can achieve girdle roundness tolerances measured in micrometres. Despite the automation, the fundamental physics remain unchanged: diamond-on-diamond abrasion is still the operative mechanism, and bruting diamonds are still consumed in the process.

Some modern facilities use diamond-coated or polycrystalline diamond (PCD) wheels as an alternative to a single mounted bruting stone, particularly for very small melee diamonds where individual stone mounting is impractical. However, for larger stones — particularly those destined for premium round brilliant cutting — the traditional opposed-stone method, now CNC-controlled, remains preferred for the degree of control it affords over girdle geometry.

Relationship to Bort and Industrial Diamond

The term bort (sometimes spelled boart) refers broadly to diamond material of industrial rather than gem quality, encompassing the fine powder produced during bruting and polishing as well as small, opaque, or cryptocrystalline diamond fragments. Bruting diamonds occupy a specific functional niche within the wider category of industrial diamond use: they are discrete, identifiable stones performing a defined shaping role, as distinct from the powdered bort used on polishing wheels or the larger industrial diamonds used in drill bits and cutting tools. The distinction matters in trade accounting, as bruting stones are tracked as tooling consumables rather than gem rough inventory.

In the Trade

Within the cutting industry, bruting diamonds are not individually graded or certified by gemological laboratories; they are traded in parcels by weight, assessed informally for size suitability relative to the work stones they will shape. A bruting diamond must be of comparable or slightly larger diameter than the stone it is rounding, since the geometry of the contact determines the efficiency of material removal. Manufacturers maintain stocks of bruting material as a routine operational input, sourcing it from rough dealers who segregate non-gem-quality production for industrial channels.

The economics of bruting diamond consumption have shifted over recent decades as synthetic diamond — produced by high-pressure, high-temperature (HPHT) or chemical vapour deposition (CVD) methods — has become available in industrial grades at competitive cost. Synthetic diamond bort and polycrystalline compacts have largely displaced natural bort in many polishing applications, though the use of natural low-grade rough as bruting stones in traditional opposed-stone bruting persists, particularly in smaller artisanal cutting operations.