The suit signs on a standard deck are a French design of the later fifteenth century, made to be printed cheaply, and the English names for two of them are mistranslations of a kind.
Carreau, not diamant
In French the sign is carreau, which means a square paving tile or a lozenge shape, not a gemstone. The name describes the shape on the card and nothing else. English chose diamond, which describes the same shape by a different comparison, and the gemstone associations that follow from the English word are a consequence of the translation rather than of the design.
A printer's solution
The French pattern replaced the pictorial German suits of hearts, bells, acorns and leaves, and the Latin suits of cups, coins, swords and batons, with four flat silhouettes in two colours. Simple shapes could be stencilled instead of engraved, which made French-suited packs cheaper to produce, which is the main reason this pattern and not another became the international standard.
The correspondence
Cartomancers map this sign onto the coin suit of the older Latin packs, which is why readings of the suit turn on money, work and material circumstances. The mapping is a convention of reading, and a reasonable one given that the German bells and the Latin coins both occupy the same slot, but it is a convention rather than a line of descent.