How Diamond Mining Methods Shape Cost Per Carat for Investors

Picture the satellite view of Botswana’s Jwaneng mine: vast terraces cut into red earth, spiralling down toward a pit floor most people would recognise instantly as diamond mining. That image is accurate for exactly one extraction method.

The other three look nothing like it. And the differences between them are where diamond production economics actually diverge.

For a resource investor, this matters more than it first appears. The same rough diamond recovered from a Namibian seabed versus a Botswanan open pit carries a fundamentally different cost structure, a different sensitivity to ore grade, and a different realised price per carat. Method is not background detail. It is the analytical foundation for reading any diamond producer’s numbers.

Here is what you will take from this: after reading it, you will be able to open any diamond mining company’s operational disclosure and immediately understand how its chosen extraction approach shapes the cost-per-carat economics that decide whether an operation survives a price downturn or profits through one.

Four methods, one commodity: how diamond extraction splits into distinct economic categories

There are four ways to get diamonds out of the ground, or in one case, out of the sea. None of them is a preference. Each is an economic response forced by the geology of the deposit itself.

The choice is imposed, not selected. And that single fact reframes how you should read a producer’s operations.

• Open-pit: Used where the diamond-bearing rock (a kimberlite pipe) sits near the surface and is large enough to justify enormous earthmoving. Highest volume, lowest unit cost at scale.
• Underground: Used when a pit gets too deep to keep stripping away surface waste economically. Lower volume, higher per-carat cost, and a major capital decision.
• Alluvial: Used where rivers have eroded diamonds from their original source and scattered them across sediment. Minimal capital, but wildly unpredictable grade.
• Marine: Used off Namibia’s coast, where ancient rivers dumped diamonds onto the seabed. High-technology, high-cost, and the highest quality stones in the industry.

Method selection is an economic choice imposed by geology, not a preference a company exercises. Where the diamonds sit determines how they must be recovered, and that in turn sets the cost structure before a single carat is sold.

The investor consequence lands before production even begins. Method determines capital intensity, cost structure, and how exposed the operation is to changes in ore grade.

Ore grade, measured in carats per hundred tonnes (cpht), is the variable that cuts across all four methods. It tells you how many carats you recover per unit of rock processed. What differs is how brutally each method punishes a grade shortfall.

Scale changes the picture too. Major producers like De Beers and ALROSA operate across several methods and geographies, blending their cost structures. ALROSA alone produced roughly 33.1 million carats in 2024 (a figure that remains unverified), spreading risk across open-pit, underground, and alluvial output.

A junior miner concentrated on a single method has no such buffer. It amplifies both the upside when grade delivers and the downside when it does not. On top of method, you are also weighing jurisdiction, water and energy access, and infrastructure, but method is the layer everything else sits on.

Open-pit and underground: the volume-versus-depth trade-off that shapes producer financials

Open-pit mining wins on one thing: throughput. Strip away the surface waste, expose the kimberlite, and process ore in bulk. The more carats you recover, the more thinly your fixed processing costs spread, which drives unit cost down.

Jwaneng in Botswana shows what a strong open-pit ore body looks like, with historical grade context around 125 cpht (unverified). That is a lot of carats per tonne, and it is why open-pit operations at scale sit at the low end of the cost curve.

Then the pit gets deeper. Haulage distances grow, the ratio of waste to ore climbs, and per-carat costs creep up. Eventually the operator faces a decision that is not a continuation of open-pit mining at all. It is a structural cost event.

Going underground changes the mine’s entire operating profile. Fixed costs rise, operational flexibility narrows, and if grade disappoints, there is far less room to adjust.

The capital risk in that transition is real, not theoretical. Lucara’s Karowe Underground Project in Botswana is a case in point.

Karowe’s total capital cost estimate rose from a feasibility figure of US$513-534 million to a revised US$683-779 million, an escalation of roughly 25-45% (all figures unverified). Transition-stage mines are where budgets move most, and where market values can diverge most sharply from disclosed reserves.

Lucara’s Karowe underground project financials confirm the scale of that escalation, with a revised total capital forecast of US$683 million disclosed in the company’s Q2 2024 management discussion and analysis, against an original feasibility estimate in the low US$500 million range.

The Venetia transition: what a maturing South African kimberlite tells investors

De Beers’ South African production now leans almost entirely on one asset: the Venetia underground operation. That concentration is the story.

Venetia’s remaining open-pit reserves have thinned to roughly 3.4 Mt at 23.6 cpht (unverified), a small tail of surface material. The underground reserve base dwarfs it, at approximately 68.9 Mt grading 77.7 cpht for around 53.6 Mct of contained diamonds (all unverified).

Treat those numbers as order-of-magnitude illustrations, not audited disclosures. The point they make is structural: De Beers’ South African future is tied to how one deep mine performs on grade and cost.

There is no open-pit buffer left to absorb a shortfall. Any grade underperformance underground flows straight into group-level unit cost. For context on how underground costs evolve, ALROSA’s historical open-pit cost sat near US$35/ct in 2016 against underground at US$38/ct in 2015, while Venetia’s 2025 unit cost is put around US$110/ct (all unverified), showing how a maturing underground operation carries a heavier per-carat load.

Producer-level financial losses of the scale reported in 2026 make the method cost framework concrete: when unit costs at a maturing underground operation approach US$110/ct and realised prices compress simultaneously, the operating leverage that made the mine attractive at higher prices inverts sharply.

Grade also moves year to year. Petra Diamonds’ Finsch mine reported a run-of-mine grade of 53.8 cpht for FY2026, up 15% on the prior year (unverified), a reminder that grade variability feeds directly into unit costs.

Marine diamond mining: why Namibia’s seabed delivers the highest price per carat in the industry

Start with how the deposit formed, because it explains everything about the price. Over millions of years, ancient rivers carried diamonds from inland sources toward the Namibian coast and out onto the seabed.

That journey was a filter. Mechanical abrasion during transport destroys weaker, fractured stones and leaves only the most durable crystals intact. What survives is disproportionately gem-quality.

The premium is not a marketing choice or a selection process. It is a direct consequence of how the diamonds got there.

Recovering them is another matter entirely. Debmarine Namibia runs a fleet of specialised vessels fitted with seabed crawler systems and suction equipment to lift diamond-bearing gravel from the ocean floor. That is a very different cost profile from moving earth on land.

The trade-off is explicit. High vessel operating costs and weather-related production variability weigh against consistently higher prices per carat.

In 2025, De Beers’ Namibia segment reported an average realised price of roughly US$353/ct, against Botswana at about US$150/ct and Canada at around US$100/ct (all unverified). That gap is the quality filter showing up in the accounts.

The premium is long-standing. In 2022, Namibia held the highest unit value among the world’s top 20 producers at approximately US$601/ct (unverified). Costs have risen too, from a Debmarine cash cost near US$191-202/ct in 2015-2016 to a consolidated Namibia segment unit cost of about US$244/ct in 2025 (unverified), but the price premium has held the margin.

A quirk worth knowing when you read technical reports: marine placer grade is often measured in carats per square metre (cpm2) rather than cpht, because the diamonds sit spread across the seabed rather than concentrated in rock. That makes direct grade comparison with land-based mines non-intuitive.

Marine carries its own distinct risk set:

• Weather-related production variability that can interrupt vessel operations
• Vessel maintenance cycles that take capacity offline
• Offshore regulatory and licensing obligations tied to marine resources
• Non-standard grade measurement (cpm2) that complicates like-for-like comparison

For your analysis, the takeaway is this. Marine exposure inside a producer’s portfolio acts as a partial natural hedge against price compression in lower-quality stone categories, which is why the Namibia segment’s margins can behave differently from an open-pit operation even when the broader diamond price is flat.

Grade sensitivity and cost per carat: the numbers that connect method choice to investment outcomes

Everything so far points to one variable. Ore grade is your primary analytical tool across all four methods, and the reason is simple mechanics.

Processing costs are largely fixed. It costs roughly the same to run rock through the plant whether that rock yields many carats or few, so grade decides how many carats absorb those fixed costs.

Watch what that does at a single operation. Mountain Province Diamonds’ Canadian operation illustrates the swing with real numbers.

When recovered grade fell by about 25%, cash costs climbed to US$132/ct. As grade later recovered, costs dropped back to US$59/ct (both figures unverified). Same mine, same method, and a more than doubling of unit cost driven almost entirely by grade.

Now scale that logic up. The same dynamic operates at group level, even for the industry’s largest producer.

De Beers deliberately processed lower-grade stockpiles as it managed volumes down. Group unit cost rose from US$71/ct in 2023 to US$93/ct in 2024, then eased to US$86/ct in 2025 as production was cut from 24.7 Mct to 21.7 Mct (all figures unverified).

That tells you something important. Even a diversified multi-method producer cannot fully insulate its per-carat economics from grade decisions at individual mines. Which is why the quality of a company’s grade disclosure is a reasonable proxy for how transparent its management is.

The natural diamond price trajectory since the 2022 peak is itself part of the context for reading producer cost disclosures: a structurally lower price environment compresses the margin available at every point on the cost curve, making method-driven cost differences more consequential than they were in a rising market.

Alluvial mining sits apart in this framework. Capital costs are minimal because the work relies on manual labour, sluices, and basic screening rather than heavy machinery.

The problem is that scattered river deposits lack the predictable shape of a kimberlite pipe, so grade estimation is structurally unreliable. Layer on governance, traceability, and smuggling risks, and most alluvial production falls outside the mainstream investable universe for institutional investors.

Reading any diamond disclosure comes down to three steps in order:

1. Identify the method. It tells you the cost structure and how much a grade miss will hurt.
2. Locate the ore grade trend. Is it improving or declining against the prior period?
3. Map the grade movement to unit cost direction. Falling grade should mean rising cost per carat, and if it does not, ask why.

That sequence turns the method-by-method mechanics into a practical filter. Grade trends are your first line of analysis, and method context is what makes those trends interpretable.

Reading a diamond producer’s disclosure after understanding the method

You started this with a satellite image. You finish it with a working analytical lens.

Three ideas hold the framework together. Method determines the cost structure. Grade determines whether that structure produces margin or losses. And producer scale, single-method versus multi-method, determines how much buffer exists when one operation stumbles.

There is a demand-side variable sitting underneath all of it. Wholesale laboratory-grown diamonds are priced around US$114/ct (unverified), which turns natural producer cost floors from an academic benchmark into a competitive threshold. Understanding a producer’s cost per carat is now partly about knowing which operators can defend their economics against a structurally cheaper substitute.

Lab-grown diamond economics now set an effective cost floor for natural producers: when wholesale synthetic stones trade near US$114/ct, any natural operation whose per-carat cost sits above that threshold cannot compete purely on price and must justify the premium through quality, provenance, or stone size.

The evidence that producers manage this actively is already visible. De Beers cut production from 24.7 Mct to 21.7 Mct between 2024 and 2025 (unverified) in response to price pressure, deliberately adjusting grade and volume rather than simply riding the market.

When you open the next operational disclosure, look for three things:

• Ore grade trend: Is it improving or declining against the prior period?
• Method-specific cost per carat: How does it compare with the previous reporting window?
• Any transition from open-pit to underground: This signals a structural cost shift ahead, and elevated execution risk.

You do not need to memorise the figures. You need to know what to look for and why it matters, and now you do.

This article is for informational purposes only and should not be considered financial advice. Investors should conduct their own research and consult with financial professionals before making investment decisions. Many of the operational and financial figures referenced above are unverified and should be treated as illustrative order-of-magnitude data, not confirmed disclosures. Past performance does not guarantee future results.