diamond

diamond a precious stone consisting of a clear and colourless crystalline form of pure carbon, the hardest naturally occurring substance, taken as a type of brilliance or excellence. The word is recorded from Middle English, and comes via Old French from medieval Latin diamas, diamant-, variant of Latin adamans, from the base of the English word ‘adamant’.

Diamond is also used for a figure with four straight sides of equal length forming two opposite acute angles and two opposite obtuse angles, a rhombus, and diamonds are thus one of the four suits in a conventional pack of playing cards, denoted by a red figure of such a shape.

Diamond was the name of Isaac Newton's dog, which according to a (probably apocryphal) story knocked over a candle which set fire to some papers and thereby destroyed the finished work of some years.
Affair of the Diamond Necklace a political scandal in 18th century France. Queen Marie Antoinette was said to have purchased a valuable necklace and subsequently denied any knowledge of the matter. Although the affair was an attempt by a French adventuress to acquire the necklace by a pretence of acting on behalf of the Queen, and Marie Antoinette was not involved, the Queen's innocence was not believed, and the scandal contributed materially to her unpopularity.
diamond cuts diamond only a diamond is hard enough to cut another diamond, used of persons who are evenly matched in wit or cunning. The saying is recorded from the early 17th century, but an earlier related usage is found in Nashe's Christ's Tears (1593), ‘An easie matter is it for any man to cut me (like a diamond) with my own dust.’
diamond jubilee the 60th anniversary of a notable event, especially a sovereign's accession or the foundation of an organization.
Diamond Sculls an annual single-scull race at Henley Royal Regatta, instituted in 1844, for which the prize was a gold pin ornamented with gold sculls and a drop diamond.
Diamond State an informal name for the state of Delaware, said to be so named because it was seen as small in size but of great importance.
diamond wedding the sixtieth anniversary of a wedding.

See also rough diamond.