Origins
The word 'diamond' entered English in the fourteenth century from Old French 'diamant,' which descended from Medieval Latin 'diamantem' (accusative of 'diamas'), itself a corruption of Latin 'adamantem' (accusative of 'adamās'). The Latin form was borrowed from Greek 'adámas' (ἀδάμας, genitive 'adámantos'), meaning 'unconquerable, invincible,' and by metonymic extension, 'the hardest substance known' — whether that was understood as a type of steel, a gemstone, or a mythical material varied by period and author.
The Greek 'adámas' is transparently composed of the prefix 'a-' (not, un-) and the stem 'damā-,' related to the verb 'damân' (δαμᾶν, to tame, to subdue, to conquer). The verb traces to PIE *demh₂- (to tame, to domesticate, to constrain), which produced English 'tame' (from Old English 'tam,' from Proto-Germanic *tamaz, from *demh₂-), Latin 'domāre' (to tame, whence English 'domestic,' 'dominate,' 'indomitable'), and Sanskrit 'dāmyati' (he is tamed). The etymological meaning of 'adámas' is thus 'un-tameable, un-conquerable' — that which cannot be subdued or broken.
In early Greek usage, 'adámas' did not necessarily refer to the gemstone now called diamond. Hesiod (c. 700 BCE) uses 'adámas' to describe an impossibly hard metal from which the sickle of Kronos was forged. Plato uses it metaphorically for anything unyielding. The identification of 'adámas' specifically with the crystalline carbon gemstone developed gradually, and Pliny the Elder (1st century CE) was among the first Latin authors to describe 'adamas' as a transparent precious stone of extraordinary hardness.
French Influence
The transformation of 'adamantem' into 'diamantem' in Medieval Latin is a case of phonological reanalysis. The initial 'a-' of 'adamantem' was reinterpreted as a prefix and dropped, or alternatively, the 'd' was doubled by dissimilation, producing forms like 'diadamas' that were then simplified to 'diamas.' The Old French 'diamant' reflects this medieval corruption, and English 'diamond' inherits it. The older, uncorrupted form survives in English 'adamant' (unyielding, inflexible) and 'adamantine' (of or like diamond, unbreakable).
The gemological and commercial history of diamonds intersects with the word's history at several points. Diamonds were known in India from at least the fourth century BCE and were traded westward through Hellenistic and Roman commercial networks. The Sanskrit word for diamond, 'vajra' (thunderbolt, also diamond), reflects a different metaphorical tradition — diamond as divine weapon rather than unconquerable substance. Medieval Europeans valued diamonds for their supposed magical and medicinal properties as much as for their beauty; they were believed to protect against poison, disease, and evil spirits.
The modern diamond industry, dominated from the late nineteenth century by De Beers, transformed 'diamond' from a word associated with rarity and aristocratic wealth into a mass-market consumer term. The 1947 advertising slogan 'A Diamond is Forever' (created by copywriter Frances Gerety for the N. W. Ayer agency) is considered one of the most effective marketing phrases ever devised, permanently linking diamonds with romantic commitment in anglophone culture.
Greek Origins
The word's derivatives in English include 'diamond' as an adjective (diamond-shaped, diamond-hard), the heraldic term 'diamond' for the lozenge shape, the baseball 'diamond' (the infield, from its shape), and the card suit 'diamonds.' The playing-card suit name dates to the sixteenth century and refers to the shape of the pips, not to the gemstone. In all its uses, the word retains an echo of its Greek etymon — the quality of being unbreakable, supreme, and enduring.