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Material World
Rock

Kimberlite

A volatile-rich volcanic rock erupted from mantle depths at extreme speed — the only rock that delivers diamond to the surface intact.

Ore

Kimberlite is the rock that brings diamonds up. It occurs as narrow carrot-shaped pipes rather than as flows or sheets, and a productive pipe may be only a few hundred metres across at the surface — which is why diamond mines are deep, narrow holes rather than the broad pits that other ores are worked from.

As a rock it is unremarkable to look at: a dark, altered, fragmental material dominated by olivine, usually weathered to a soft blue or yellow ground near the surface. Its importance is entirely in what it carries. Kimberlite does not make diamonds; it samples them, tearing fragments from mantle rock on its way through and arriving with them still intact.

How it forms

Diamond is only stable at depths greater than about 150 kilometres. Bring it up slowly, or hot, and it converts to graphite — the form of carbon that is stable at the surface, and the form every diamond on Earth is slowly trying to become.

What makes kimberlite unique is the speed of its ascent. Charged with carbon dioxide and water, the magma expands as it rises, accelerating rather than slowing, and reaches the surface in a matter of hours at speeds that have been estimated in tens of kilometres per hour. The diamonds it carries have no time to re-equilibrate. They arrive as passengers, not products, which is why diamond is not a constituent of kimberlite in any ordinary sense.

The pipes are also old: most cluster in ancient continental cratons, and the eruptions that formed them have no observed modern equivalent.

Economic significance

Diamond grades are quoted in carats per hundred tonnes, and a rich pipe might run a few tens of carats — meaning the rock is, by mass, some parts per billion diamond. Almost the entire value of a diamond mine sits in a fraction of the ore too small to see without processing it.

That is also why exploration for kimberlite is done indirectly, by looking for the durable indicator minerals that survive weathering out of the pipe and travel downstream: chromian garnet, chrome diopside, picroilmenite. Finding those in a stream sediment is the standard route to finding the pipe they came from.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is composed of

  • Olivine — mineral · 40–70% · the mineral the rock is mostly made of, generally altered to serpentine near the surface
  • Forsterite — mineral · the magnesian end of the olivine series, which is what mantle olivine is — the same crystals as the group edge above, named more precisely
  • Diamond — mineral · not a constituent but a xenocryst torn from mantle rock during ascent — present at grades measured in parts per billion, and carrying essentially all the deposit's value
  • Ilmenite — mineral · as magnesian picroilmenite, one of the indicator minerals used to trace a pipe from the stream sediment downstream of it

is sourced from

  • Peridotite — rock · generated far deeper in the mantle than ordinary basalt, and carrying fragments of the peridotite it passed through as xenoliths

Sources

  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Kimberlite back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Kimberlite → is composed of (not a constituent but a xenocryst torn from mantle rock during ascent — present at grades measured in parts per billion, and carrying essentially all the deposit's value) → Diamond → is produced by (as CVD diamond, grown from methane and hydrogen in a plasma at a pressure far below the one diamond was supposed to need — and chemically and structurally identical to the mined mineral) → Vapour deposition
  • Kimberlite → is sourced from (generated far deeper in the mantle than ordinary basalt, and carrying fragments of the peridotite it passed through as xenoliths) → Peridotite → is composed of (the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition) → Olivine
  • Kimberlite → is composed of (the mineral the rock is mostly made of, generally altered to serpentine near the surface) → Olivine
  • Kimberlite → is composed of (the magnesian end of the olivine series, which is what mantle olivine is — the same crystals as the group edge above, named more precisely) → Forsterite
  • Kimberlite → is composed of (as magnesian picroilmenite, one of the indicator minerals used to trace a pipe from the stream sediment downstream of it) → Ilmenite
  • Kimberlite → is sourced from (generated far deeper in the mantle than ordinary basalt, and carrying fragments of the peridotite it passed through as xenoliths) → Peridotite → is composed of (the magnesian end of the olivine series, and what mantle olivine actually is — around ninety per cent forsterite in most peridotite) → Forsterite

These are the most distinct paths back. Kimberlite can be traced through others besides.