Diamond

Diamond is pure carbon in a cubic crystal structure, in which every atom is covalently bonded to four neighbours in a rigid tetrahedral framework. That structure is the entire explanation of its properties: extreme hardness, very high thermal conductivity, high refractive index and strong dispersion. Graphite is the same element in sheets, and is one of the softest materials known. The difference between the hardest and one of the softest substances in nature is arrangement, not composition.

Diamond sits at 10 on the Mohs scale, which is a ranking of scratch resistance and not a linear measure, the gap between 9 and 10 is far larger than between any other adjacent pair. Hard is not the same as tough: diamond has perfect cleavage in four directions and can be split cleanly with a well-placed blow, which is how diamonds were shaped before modern sawing.

The Greek adamas, unconquerable or unbreakable, is the root of both diamond and adamant. The word was not originally specific to this stone and was applied to any exceptionally hard material, which is another instance of an ancient gem name that cannot be assumed to mean what it now means. Pliny's account of adamas includes claims, that it resists blows on an anvil, that it is softened by goat's blood, that do not describe diamond accurately, and the goat's blood story travelled through the lapidary tradition for well over a thousand years.

Until the eighteenth century essentially all diamonds came from India, principally the Golconda region; Brazilian discoveries followed, and the South African finds from the late 1860s changed the scale of the trade completely.

The diamond engagement ring is routinely described as an ancient custom. Its modern form is a documented advertising campaign.

Diamond rings as betrothal tokens do have earlier precedent among European nobility, conventionally traced to the ring Maximilian of Austria gave Mary of Burgundy in 1477, and diamonds appear in aristocratic marriage gifts thereafter. But diamond engagement rings were not a general practice, and in the 1930s the diamond market was weak and the stone was not the default.

De Beers, which controlled the great majority of world rough diamond supply, engaged the advertising agency N. W. Ayer in 1938. The campaign that followed created the slogan A Diamond Is Forever in 1947, promoted the idea that a man should spend a defined multiple of his salary on a ring, a figure that was itself raised over the decades, and placed diamonds systematically in film and on public figures. The proportion of American brides receiving diamond rings rose from a minority to a large majority over the following decades. The campaign is one of the most studied in the history of advertising and De Beers has never particularly disputed the account.

The permanence slogan also solved a specific commercial problem: a stone that is never resold has no secondary market to depress prices. The scarcity, likewise, was substantially managed rather than natural, through supply control that persisted until the cartel's structure weakened in the 2000s.

Vedic tradition assigns diamond, heera, to Venus, Shukra, and it is one of the navaratna. That attribution is genuinely older than the advertising, comes from a different civilisation, and has nothing to do with marriage in the Western sense.