Rough Diamond Grading: How Buyers Assess and Price Your Material

Why rough grading is different from polished grading

The 4Cs — cut, colour, clarity, and carat weight — are the standard framework for grading polished diamonds, as established by the Gemological Institute of America (GIA) and adopted globally. A GIA Diamond Grading Report tells you, with high precision, the exact colour grade, clarity grade, cut grade, and weight of a finished, polished stone. These grades directly correlate with market prices via the Rapaport Price List.

Rough diamond grading works on an entirely different basis. A rough stone has not yet been cut or polished, so it has no cut grade and no definitive polished colour or clarity grade. What it has is a set of physical characteristics — external shape, visible colour, internal features observable through the rough surface, and weight — that allow an experienced buyer to estimate what the stone might yield after cutting and polishing. Rough grading is not a measurement of what the stone is; it is an informed estimate of what the stone could become.

This distinction has direct commercial consequences. The same rough diamond will be assessed differently by different buyers, because yield estimation is a skill that varies with experience and manufacturing capability. A Mumbai manufacturer with a highly efficient cutting operation will extract more value from a given piece of rough than a smaller workshop with less capable equipment. This is why rough diamond prices — unlike polished prices — are not published in a simple, universal price list. They are negotiated, and the spread between buyers reflects the variance in their yield assumptions.

For sellers, the implication is clear: understanding how your material will be assessed — what characteristics buyers look at, and why — allows you to present your parcel more professionally, obtain a more credible valuation, and negotiate from a position of knowledge rather than uncertainty.

Sieve sizing — how rough is measured

Rough diamonds are not weighed individually and then sorted by weight at scale — the process would be prohibitively slow for large parcels. Instead, rough is mechanically sieved through a series of screens with defined aperture sizes. Each screen has a specific number of openings per linear inch; a stone that passes through a coarser screen but is retained by a finer screen falls into a defined sieve category. The notation uses the convention "-X+Y" meaning: passes through sieve X, retained by sieve Y.

Common sieve categories used in the rough diamond trade and their approximate carat-per-stone equivalents: the -3+2 category (tiny material, approximately 0.01–0.03 carats per stone) is near-industrial; -4+3 (approximately 0.03–0.07 ct) is the smallest gem-potential category; -6+4 (approximately 0.07–0.18 ct) represents small gem rough; -8+6 (approximately 0.18–0.40 ct) is a significant volume category for African run-of-mine material; -11+8 (approximately 0.40–1.0 ct) is where premium pricing begins; -14+11 (approximately 1.0–2.0 ct) is high-value territory; stones above +14 (approximately above 2.0 carats as rough) are priced individually and assessed stone by stone rather than by parcel average.

The commercial significance of sieve size is dramatic. A parcel graded predominantly -8+6 will be priced at a fraction of an otherwise equivalent parcel graded predominantly -14+11. The larger the average stone size, the higher the expected polished yield per stone, the better the cut grade potential, and the higher the polished market value per carat. Price per carat rises non-linearly with size — a 2-carat polished round brilliant does not sell for twice the price of a 1-carat equivalent; it sells for three to five times as much, depending on quality. This premium is embedded in the rough price from the outset.

When presenting a parcel to buyers, have your sieve distribution measured before the meeting. This is done by your carrier, a licenced dealer, or a professional valuer. The distribution should be expressed as the percentage by carat weight falling into each sieve category — for example, "40% in -11+8, 35% in -8+6, 15% in -14+11, 10% in -6+4 and below." A buyer who sees this number knows the approximate polished yield profile of your parcel before touching it.

Shape categories

The crystallographic shape of a rough diamond determines the efficiency of the cutting process and, therefore, a significant portion of its rough market value. Shapes are not uniform; rough diamonds crystallise in multiple forms, and a single run-of-mine parcel typically contains a distribution of all of them.

Octahedra are the most commercially valuable shape category. An octahedron is the classic eight-faced double-pyramid form — the diamond shape of popular imagination, though the rough stone is not yet transparent or brilliant. The octahedral form is geometrically efficient for cutting a round brilliant: the cutter places the table facet at one apex and works down to the girdle, recovering approximately 45–55% of the rough weight as a polished round brilliant. The shape's symmetry also allows the cutter to assess the internal quality before committing to a cutting plan.

Dodecahedra (twelve-faced forms, rounded in appearance) yield round brilliants and fancy shapes at somewhat lower efficiency — typically 35–45% recovery — because the rounded faces require more material removal before a polished girdle can be established. They are commercially valuable but priced below equivalent octahedra.

Macles are twinned crystals — two octahedral crystals joined at a common face, producing a flattened, triangular form. Macles have specialist cutting applications (rose cuts, large tables, and some fancy shapes) but cannot be efficiently cut into round brilliants. They are priced at a discount to octahedra of equivalent colour and clarity. Some jewellery designers specifically seek macles for bespoke applications; outside that niche, expect a 20–35% price discount versus octahedra.

Cleavages are diamond pieces that have broken along natural cleavage planes — either in the earth or during mining. The break may be clean or irregular. Cleavages have polished potential but carry additional manufacturing risk: the cutter cannot know with certainty whether additional cleavage planes are present. They are priced accordingly, typically at 30–50% of octahedral equivalents. Chips are irregular fragments, typically the result of mechanical impact during mining. Polished potential is limited; chips often trade at near-industrial prices.

Colour in rough

Colour is assessed in rough diamonds under standardised lighting conditions — a D65 daylight-equivalent source, neutral background, consistent viewing angle. The assessor is looking at the body colour of the rough stone through its surface. This is more challenging than grading a polished stone because the rough surface scatters and diffracts light differently; experienced rough graders compensate through pattern recognition built over many thousands of stones.

The rough colour assessment maps, approximately, to the GIA polished colour scale. Rough assessed as "top cape" or better is expected to yield polished stones in the D–F range after cutting and polishing removes the graining and surface irregularities that affect rough colour perception. "Cape" material is expected to yield G–J range. "Off-colour" rough — visible yellow or brown body colour — maps to K and below.

The key word is "approximately." Rough colour prediction is not precise; colour graining patterns, inclusions, and the geometry of the rough can cause the polished outcome to diverge from the rough assessment. Professional buyers apply a risk discount to rough colour assessments, particularly for larger stones where the polished outcome has a higher USD consequence.

Fluorescence is assessed separately. Strong blue fluorescence in rough typically indicates stones that will exhibit blue fluorescence under UV in polished form — which, depending on the polished colour grade, may be commercially neutral or create a slight discount versus non-fluorescent equivalents. Rough buyers who intend to sell into the GIA grading pipeline will discount strongly fluorescent material accordingly.

Run-of-mine African material — unsorted, direct from mine — typically presents as mixed colour. Professional buyers purchasing run-of-mine material price against the colour average, applying a discount for the spread. Sorted gem parcels, where a licenced dealer has removed the best-colour stones, carry a colour premium but should be presented with documentation that the sorting was conducted before valuation — buyers will ask.

Clarity in rough

Clarity in rough diamonds is assessed under 10× loupe magnification. The categories used in rough trading are broader than the GIA polished clarity scale; rough clarity assessment is a commercial classification rather than a scientific measurement, because the interior of a rough stone cannot be fully assessed without cutting it open.

The three commercial clarity categories are gem, near-gem, and industrial. Gem rough shows no internal inclusions visible under 10× loupe from the assessor's view of the stone. It is expected to yield polished stones at VS or better clarity, though the expectation is not a guarantee. Near-gem rough has visible inclusions under 10×, but the assessor judges that cutting around the inclusion or cleaving the stone will yield a polished stone of commercial gem quality from at least part of the rough. Industrial rough is heavily included — boart, fibrous, or shattered — and its commercial application is abrasive and drilling uses rather than gem cutting. Industrial diamonds price at USD 1–10 per carat, versus USD 100–50,000-plus per carat for gem quality.

The proportion of gem to near-gem to industrial in a parcel is the most important single driver of parcel average price. A parcel that is 80% gem, 15% near-gem, and 5% industrial is priced dramatically higher than a parcel that is 40% gem, 30% near-gem, and 30% industrial, even if the sieve distribution and colour are identical.

The DTC (Diamond Trading Company) system

The Diamond Trading Company — the marketing arm of De Beers, now integrated into the De Beers Group — developed the classification system that became the de facto industry standard for rough diamond description. The DTC system categorises rough into boxes by quality type (gem, near-gem, industrial), size range, shape, and colour. Each box has a reference value based on the DTC's own price book, which is revised periodically based on market conditions.

While the DTC price book is not publicly published, its framework is well understood by the industry. Antwerp buyers, Mumbai manufacturers, and Dubai traders all reference DTC box descriptions when discussing rough parcels — "it is a -11+8 gem octahedral near-colourless parcel" conveys, to any professional buyer, a precise picture of the material and an approximate USD/ct expectation.

Botswana, through the Okavango Diamond Company (ODC, the Botswana government's rough marketing entity), Namibia through Namdia, and South Africa through the state diamond trader all use DTC-compatible classification for their production. For sellers in these countries, the national KP valuation process uses DTC categories, which means your material arrives at the international market with a recognised, professionally applied classification.

How Antwerp buyers grade parcels

The Antwerp buying room — whether in a bourse or at a private trader's office — is where rough parcel assessment converges into a price. An experienced Antwerp buyer can assess a parcel of several hundred carats within 20–30 minutes. The process is systematic: the stones are spread on a white grading cloth under standard lighting, and the buyer works through the parcel assessing each of the four dimensions we have described — sieve distribution, shape mix, colour average, and gem-to-industrial ratio.

The buyer then applies their yield calculation. This is the proprietary element — the buyer's estimate of the polished output they can recover, based on their manufacturing relationships and experience with similar material. The calculation is: (expected polished yield in carats) × (polished market price per carat for the expected output quality) × (rough discount factor, which absorbs manufacturing cost, yield risk, and the buyer's margin requirement). The result is the buyer's maximum offer per carat for the rough parcel.

The rough discount factor varies. For well-sorted, well-documented, high-quality African gem rough, it ranges from 40–55% of expected polished value — meaning the buyer pays 45–60 cents per dollar of expected polished output. For run-of-mine material with mixed quality, the factor is wider — buyers pay 25–40% of expected polished value, reflecting the higher yield uncertainty. For exceptional individual stones above 10 carats, the calculation is done stone by stone, not as a parcel average, and the discount factor is negotiated separately.

Pricing a parcel — worked example

To make the calculation concrete, consider a 100-carat parcel of sorted African gem rough with the following characteristics: 70% gem quality, 20% near-gem, 10% industrial (by carat weight). Sieve distribution: 45% in -8+6, 35% in -11+8, 10% in -6+4 and below, 10% above +11. Colour: near-colourless average, assessed as predominantly F–H equivalent. Shape: 55% octahedra, 25% dodecahedra, 15% macles, 5% cleavages and chips.

The buyer's first step is to separate the parcel into its quality categories for pricing. The 70 gem-quality carats are the primary value driver. Within those 70 carats, the sieve distribution skews toward -8+6 and -11+8. A typical polished yield for this size range and shape mix is approximately 40%: 70 carats of gem rough will yield approximately 28 carats of polished output. The expected polished colour, after cutting, is G–H range for the majority of the parcel. At a polished market reference price of approximately USD 3,500 per carat for a G VS2 round brilliant in the 0.30–0.50 carat range (representative of -8+6 and -11+8 rough output), the expected polished value of the gem portion is approximately USD 98,000.

The near-gem portion (20 carats of rough) yields perhaps 7–8 carats of polished output, predominantly in the SI1–SI2 clarity range, at an average polished price of approximately USD 1,800 per carat — adding approximately USD 13,500 to the expected polished value. The industrial portion (10 carats) adds USD 50–100.

Total expected polished value: approximately USD 111,600. Applying a rough discount factor of 45% (appropriate for a well-documented, sorted gem parcel from a licensed African producer), the buyer's offer is approximately USD 50,200 — or USD 502 per carat for the 100-carat rough parcel. This is the buyer's initial position. With a credible independent valuation in hand and knowledge of the parcel's characteristics, the seller can negotiate from this basis.

What sellers should know before presenting to buyers

The practical preparation for an institutional rough diamond sale requires four things in place before you enter a conversation with a qualified buyer.

First, a DTC-equivalent valuation or an independent valuation from a recognised valuer — conducted on your actual parcel, not based on comparable sales or estimates. This gives you a credible floor for negotiation and signals to the buyer that you have gone through professional assessment.

Second, your sieve distribution data — the percentage breakdown by carat weight into the major sieve categories. This is produced as part of the valuation process, or can be done separately by a licenced dealer with sieve equipment.

Third, your shape distribution assessment — the approximate percentage breakdown into octahedra, dodecahedra, macles, cleavages, and chips. Again, this is typically produced as part of the valuation.

Fourth, your Kimberley Process certificate (or confirmation that the application is in process and the expected issuance date). Without the KP cert, no buyer can proceed, regardless of material quality or price agreement.

Sellers who arrive at a buyer conversation with these four elements prepared are treated differently from sellers who do not. The professional presentation signals that you are a serious counterparty, that your material is legitimate, and that your expectations are grounded in market reality. It also reduces the buyer's diligence burden, which means they can commit faster. In a market where many sellers present documentation as an afterthought, preparation is a competitive advantage.