Kimberlite is a rare, volatile-rich igneous rock known for forming vertical, carrot-shaped structures within the Earth’s crust called kimberlite pipes. This rock is an unusual variant of peridotite, originating from extreme depths within the planet’s mantle, between 150 and 450 kilometers below the surface. The question of kimberlite’s worth is complex because the rock itself is not a commodity. Its economic significance is tied entirely to its ability to transport valuable cargo to the surface.
Kimberlite as a Diamond Host Rock
Kimberlite’s primary role is that of a high-speed elevator for diamonds, which are xenocrysts—foreign crystals—that did not form within the kimberlite melt itself. These diamonds formed billions of years ago in the stable, deep regions of the mantle under immense pressure and temperature. The kimberlite magma, rich in carbon dioxide and other volatile components, erupted rapidly and violently from these depths, carrying the diamonds toward the surface in a matter of hours.
This explosive, rapid ascent prevents the diamonds from dissolving back into carbon, preserving them for mining. The kimberlite rock itself has negligible commercial value, often only suitable for use as low-grade aggregate. The financial value of a kimberlite pipe is entirely a function of the concentration and quality of the diamonds it contains. Only about 30 kimberlite pipes worldwide are actively mined for diamonds, illustrating the rarity of economically viable deposits.
Geological Factors Determining Deposit Value
The first and most measurable metric for a kimberlite deposit’s potential worth is its diamond grade, expressed as carats per tonne (cpt) of processed rock. The median grade for a diamond-bearing kimberlite pipe is approximately 0.25 cpt, though a deposit exceeding 2.0 cpt is considered exceptionally rich. This grade directly determines the volume of rock that must be mined and processed, heavily influencing operational costs.
The total volume of the kimberlite pipe is another fundamental geological factor that translates into a resource estimate. The typical outcrop area of a diamond-bearing pipe is about 12 hectares, with a median total tonnage of around 26 million metric tons of material. However, not all geological phases within a single large pipe may be diamondiferous, meaning total tonnage does not always equate to total resource. Geologists must model the three-dimensional geometry and internal variations of the pipe to isolate the zones that contain the diamond-bearing material.
Beyond sheer weight, the quality and size distribution of the diamonds are significantly more influential on the overall worth. A deposit containing a high proportion of large, gem-quality stones is exponentially more valuable than one with the same total carat weight composed of small, industrial-grade material. The median size of recovered diamonds is small, about 0.07 carats, with a median of 67% of the total yield often being industrial quality. The presence of just a few large, high-value diamonds can dramatically increase the average dollar-per-carat value of the entire parcel, even if the overall grade is moderate.
Economic Assessment and Market Valuation
Bulk Sampling and Valuation
Initial geological assessment requires extensive bulk sampling to transition from a theoretical resource to a market valuation. This involves processing thousands of tonnes of kimberlite to recover a large parcel of diamonds. This bulk sample analysis confirms the grade and provides a crucial financial data point: the dollar value per carat, which is proprietary and constantly fluctuating.
Cost and Feasibility
The raw, in-situ value of the diamonds must then be heavily discounted by the projected costs of mining, processing, and infrastructure, determined through a detailed feasibility study. Factors like the pipe’s depth, remoteness, rock complexity, and necessary processing techniques can render a diamond-bearing kimberlite uneconomic. A remote location with a moderate grade may be worth less than a lower-grade pipe that is close to existing infrastructure.
Net Present Value (NPV)
The final worth of a kimberlite deposit is calculated using the Net Present Value (NPV), the standard financial metric for mineral assets. NPV accounts for all projected cash flows over the expected life of the mine, including revenues from diamond sales and all operating and capital expenditures. This calculation applies a discount rate, typically around 8% in the diamond industry, to future revenues to account for the time value of money and inherent risk. A positive NPV indicates that the project is likely to be profitable, providing the definitive dollar figure for the deposit’s market valuation.