Diamonds is one of the four suits of the standard French-suited playing pack, and in the tradition of cartomancy, fortune-telling with ordinary playing cards, it carries a settled range of meaning: money, work and the material world.
Suit, element and tarot parallel
When cartomancers and occultists set the playing-card suits beside the four suits of the tarot's Minor Arcana, Diamonds is paired with the tarot suit of Pentacles and with the classical element of Earth. This scheme of suit, element and tarot equivalence was systematised in the Western esoteric revival, notably by the Hermetic Order of the Golden Dawn from 1888, though the practice of reading fortunes from a common deck is older and more widespread.
The correspondence is one convention among several, other writers have exchanged the elemental attributions of two suits, and it is described here as part of the documented cartomantic tradition rather than as a fixed rule.