Gem library
Diamond
Pure carbon, the hardest natural material
Diamond is pure carbon crystallised more than 150 km down, under 45,000 atmospheres and 1,200 °C. Brought to the surface by kimberlite eruptions of astonishing violence, it is the hardest natural material known — and a textbook case of the difference between hardness and fragility.
- Hardness (Mohs)
- 10
- Density
- 3.50 – 3.53
- Crystal system
- Cubic
- Refractive index
- 2.417 – 2.419
- Cleavage
- Perfect on {111}
Mineralogical identity
- Family
- Native elements
- Species
- Diamond
- Variety
- Fancy coloured diamonds, black diamonds
- Chemical formula
- C — pure crystallised carbon
- Crystal system
- Cubic
- Classification
- Precious stone
- Rarity
- Very rare
- Indicative price
- €1,500 – 30,000/ct (fancy pinks and blues: millions/ct)
Etymology
From the Greek adamas, «unconquerable, unyielding» — via the Latin adamantem, which also gave the French word for magnet through a medieval confusion.
Physical & optical properties
- Hardness (Mohs)
- 10
- Density
- 3.50 – 3.53
- Refractive index
- 2.417 – 2.419
- Birefringence
- None (isotropic)
- Cleavage
- Perfect on {111} — octahedral
- Fracture
- Conchoidal
- Lustre
- Adamantine
- Diaphaneity
- Transparent
- UV fluorescence
- Variable (often blue)
- Pleochroism
- None
- Colours
- colourless, yellow, brown, pink, blue, green, black
Formation, geology & history
An extreme formation
Diamond crystallises in the Earth's mantle, between 150 and 250 km down, in conditions where carbon adopts a compact cubic structure instead of layered graphite. Most gem diamonds are between one and three billion years old. Their ascent is the work of kimberlites: ultra-fast eruptions, at tens of metres per second, that cross the lithosphere before the diamond has time to turn into graphite. A slower magma would destroy it. These pipes, discovered at Kimberley in South Africa in 1871, remain the main source; the alluvial diamonds of Congo, Angola and Namibia come from kimberlites dismantled by erosion.
Hard but cleavable
Hardness 10 means nothing scratches it except another diamond. But diamond has a perfect octahedral cleavage: a well-placed hammer blow splits it cleanly, a property cutters have always exploited to break down rough. A set diamond can therefore chip on a violent impact, particularly at fine points. Another paradox: diamond burns. Heated to 700-800 °C in the presence of oxygen it turns into CO₂ — a real risk during torch repairs on jewellery. Finally, its thermal conductivity is the highest of any known material, which underpins the thermal conductivity tester used by jewellers.
The 4Cs and the market
A diamond's value rests on four criteria: carat (weight), colour (from D, exceptional colourless, to Z, yellow), clarity (from FL, flawless, to I3, visible inclusions) and cut, the only human criterion. The 57-facet round brilliant, calculated by Marcel Tolkowsky in 1919, optimises light return. Intensely coloured — fancy — diamonds escape this grid: the 59.60-carat Pink Star sold for 71.2 million dollars in 2017. Since the 2010s, HPHT and CVD synthetic diamonds, chemically identical and detectable only with specialist equipment, have cut prices in that segment by 60 to 80 % and upended the market.
How to recognise it
- Thermal conductivityThe tester reading is immediate and reliable: diamond sheds heat better than any other material. Moissanite fools it, however, hence combined testers.
- Adamantine lustre and fireIndex 2.42 and high dispersion: the lustre is unique, with very lively white and coloured flashes. Zircon and moissanite have too much fire, glass not enough.
- The line testA cut diamond placed on a line drawn on paper will not let the line be seen through it (total refraction). A stone through which the line is visible is a simulant.
Imitations, treatments & confusions
- Cubic zirconia (CZ)The most widespread simulant since 1976. Denser (5.8), softer in lustre, it scratches over time and its facet edges blunt. Cost: a few euros.
- MoissaniteSynthetic silicon carbide, hardness 9.25, too much fire and visible birefringence (doubled edges under a loupe). It fools simple thermal conductivity testers.
- Synthetic diamond (HPHT and CVD)Not a fake: it is a real diamond, produced in a laboratory. Chemically and optically identical, detectable only by specialist machine. It must be declared, and is worth 60 to 80 % less.
Deposits & origins
Botswana (leading world producer by value, Jwaneng mine), Russia (Yakutia, Mir mine), Canada (Northwest Territories, Diavik, Ekati), South Africa (Kimberley, the historic deposit), Angola, Namibia (marine diamonds), Australia (Argyle, pinks, closed in 2020)
Care & precautions
- Hot soapy water, a soft toothbrush, ultrasonic cleaning: diamond tolerates everything. A little diluted ammonia revives the brilliance.
- Mind impacts on the points: perfect octahedral cleavage makes the corners vulnerable despite hardness 10.
- Never torch it without protection: diamond burns from 700 °C in the presence of oxygen.
- Have the setting claws checked every two years: most lost diamonds are lost to a failure of the mount, not the stone.
Traditions & crystal healing
- Chakras
- Crown
- Zodiac signs
- Aries · Leo · Taurus
- Element
- Fire · Ether
- Month
- April
A symbol of invincibility and unshakeable commitment, diamond is associated in tradition with absolute clarity and constancy. The Greeks saw it as splinters of stars, Indian tradition linked it to Venus. Its place in engagement rings, by contrast, is a recent commercial construction, born of a 1947 advertising campaign.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
How do I recognise a real diamond?
The thermal conductivity tester is the most reliable in practice. Failing that: a diamond does not let a line be seen through its table, does not stay misted when breathed on, and its facet edges remain perfectly crisp.
Is a lab-grown diamond a fake?
No: chemically, physically and optically it is a diamond. Only its origin differs. It must be declared as such and is worth 60 to 80 % less than an equivalent natural diamond.
Can a diamond break?
Yes. Hardness 10 means nothing scratches it, but its perfect octahedral cleavage makes it vulnerable to impact: a brilliant's point can chip against a door frame.