Diamond

What it actually is

Carbon, cubic, hardness 10, with perfect cleavage in four directions.

That hardness is not a small step above corundum's 9. The Mohs scale is ordinal rather than linear, and diamond is several times harder than corundum, which is itself several times harder than quartz.

Hardness resists scratching and says nothing about splitting. Diamond's four cleavage directions are exactly what cutters exploited before laser sawing, and a knock on the girdle at the wrong angle can chip or split a stone.

Diamond has the highest thermal conductivity of any bulk material, which is what handheld diamond testers measure.

Colour comes from impurities and structural defects: nitrogen gives yellow, boron gives blue, and plastic deformation gives pink and red. Colourless diamond is diamond with very little of anything else in it.

The name comes from the Greek adamas, unconquerable, which is also the root of adamant.

Where it comes from

Diamonds form at depths of about a hundred and fifty to two hundred kilometres, in the lithospheric mantle beneath ancient continental cores, and are carried to the surface by kimberlite and lamproite eruptions that travel very fast.

Botswana, Russia, Canada, South Africa, Angola and Australia are the major producers. Canada's Arctic mines opened from the 1990s and are often cited for traceability.

India was the world's only significant source until the eighteenth century, with the Golconda mines producing the historic stones, including the Koh-i-Noor and the Hope. Brazilian discoveries followed, and the South African finds from 1867 transformed the trade entirely.

The De Beers consortium formed in that period and controlled supply for most of the twentieth century, which is directly relevant to how the stone's meaning was constructed.

What the traditions claimed

Indian tradition is the oldest. Diamonds were mined in India for over two thousand years, and in the navaratna diamond is heera, the gem of Venus, prescribed for relationships, comfort and artistry.

Pliny records diamond as proof against poison and against madness, and medieval European lapidaries repeat protective claims and add that it loses power if acquired dishonestly.

The engagement association, however, is manufactured and its manufacture is documented. Diamonds were not the standard engagement stone before the twentieth century. In 1947 Frances Gerety, a copywriter at the N. W. Ayer agency working for De Beers, wrote the line a diamond is forever, and a sustained campaign tied diamonds to engagement first in the United States and then in Japan, where diamond engagement rings went from near zero in the 1960s to a majority within two decades.

Nothing else in this catalogue has its meaning documented from the boardroom. It is worth knowing precisely because every other stone's meaning was probably built in some comparable way, just without the paperwork.

How it is worked with

If you own one, the deep-time fact is more interesting than anything on the shop card. A diamond in a ring is between one and three billion years old and formed at a depth no human has reached.

The honest question a diamond raises is about what a symbol is worth when you know who wrote it, which is a genuinely useful reflection and one the stone is uniquely suited to.

For wear it is the most durable gem, resistant to scratching by anything, and it can be steam cleaned. Watch the girdle for chips, and have settings checked.

For buying, the substantive questions are laboratory-grown versus mined, and provenance. Laboratory-grown diamond is real diamond at a fraction of the price. The Kimberley Process addresses conflict financing but not labour conditions or environmental impact, and its adequacy is disputed.

Buying it honestly

Laboratory-grown diamond is diamond, produced by high-pressure high-temperature or chemical vapour deposition methods, and it must be disclosed as laboratory-grown. It is not a simulant and it is not a fake. The price difference is large and growing.

Cubic zirconia and moissanite are simulants: different materials that look similar. Moissanite has higher dispersion and shows doubled facet edges under magnification, and both fail a thermal conductivity test that diamond passes, though moissanite fools some older testers.

Clarity enhancement by fracture filling and laser drilling exists and must be disclosed.

Treated colour, including irradiated blue and green and HPHT-treated colourless, is common and must be disclosed.

For anything expensive, insist on a report from a reputable laboratory, and read what it says about origin and treatment rather than only the grades.