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Material World
Mineral · C

Diamond

Carbon in a cubic lattice — the hardest naturally occurring substance, and one of the best thermal conductors known.

Gemstone Abrasive

Diamond is carbon arranged in a cubic lattice in which every atom is covalently bonded to four neighbours in three dimensions. That structure is the whole story: it is what makes diamond the hardest natural substance, an excellent thermal conductor, and an electrical insulator despite being a pure element.

Most diamond by mass is industrial, not ornamental. Its value in cutting and grinding is more economically significant than its value in jewellery, though not more visibly so.

Why it behaves as it does

Diamond is hard because of how its carbon atoms are joined, not because of what they are. Each atom is covalently bonded to four neighbours in a rigid tetrahedral arrangement that repeats in every direction, so a scratch has to break covalent bonds rather than slide one part of the structure past another. There is no plane through the crystal along which the bonding is weak.

Graphite is the same element and is one of the softest minerals known, which is the clearest possible demonstration that the hardness belongs to the arrangement. Graphite bonds strongly within flat sheets and only weakly between them, so the sheets shear apart under a fingernail and the surface feels slippery — the structure that makes diamond unscratchable and the structure that makes graphite a lubricant are built from identical atoms.

The same rigidity explains diamond's thermal conductivity, which is higher than any metal's. Heat travels through a solid as lattice vibrations, and a stiff, light, near-perfect lattice carries them exceptionally well.

How it forms

Natural diamond forms at depths of 150 kilometres or more, where pressure is high enough for the cubic form to be stable, and is brought to the surface rapidly by kimberlite eruptions. Slow ascent would allow it to revert toward graphite; it survives only because the journey is violent.

How to identify it

Adamantine lustre, extreme hardness — it scratches every other natural material and is scratched by none — and very high thermal conductivity, which is what handheld diamond testers actually measure. A diamond drains heat from a fingertip noticeably faster than any imitation.

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 a polymorph of

  • Graphite — mineral · both pure carbon; cubic versus hexagonal layered structure Wikidata

contains

is used as

  • Abrasive — application
  • Jewellery and adornment — application · and the market is close to the whole of diamond's value while being a small fraction of its tonnage — most diamond mined is industrial grade and ends up on a saw blade

is commonly confused with

  • Graphite — mineral · identical composition, opposite properties — the standard illustration of why structure matters
  • Zirconia — compound · which is the entire commercial point of cubic zirconia, and it is separated instantly by thermal conductivity, which is what a jeweller's tester measures

is an alternative to

  • Zircon — mineral · as a gemstone only: zircon's dispersion and adamantine lustre made it the traditional diamond substitute, long before synthetic stones. It is nowhere near as hard
  • Silicon carbide — compound · as an abrasive: diamond cuts what silicon carbide cannot, including silicon carbide itself, and synthetic diamond made the comparison one of cost rather than of availability
  • Tungsten carbide — compound · as a cutting material: carbide is tougher and far cheaper, diamond is harder and cuts what carbide cannot
  • Corundum — mineral · as a hard synthetic crystal: both are grown industrially, and sapphire is used wherever diamond's hardness is more than the job needs and its price more than the budget
  • Zirconia — compound · as a gemstone, and cubic zirconia is the reason a diamond simulant is a settled question — hard enough to wear, disperses more light, and about three hundred times cheaper

is a component of

  • Kimberlite — rock · 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

is produced by

  • Vapour deposition — process · 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

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Crystallography Open Database
    COD Advisory Board / Vilnius University · CC0 — contributors place data in the public domain

Questions this page answers

Where it comes from, and what it becomes

Follow Diamond 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

  • 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

Downstream — what it becomes

  • Diamond → is used as → Abrasive
  • Diamond → is a component 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) → Kimberlite
  • Diamond → is used as (and the market is close to the whole of diamond's value while being a small fraction of its tonnage — most diamond mined is industrial grade and ends up on a saw blade) → Jewellery and adornment