The diamond suit in a pack of cards is thought to have evolved from another shape in use over 500 years ago in France.
Playing cards are a lot older than the game of poker, almost a thousand years older, and they originated in China. But because of trading routes like the silk road, cards made their way to the middle east and then to Europe by the middle of the last millennium, if not earlier than that. They came via Egypt. Swords, cups, coins and polo-sticks were the suits used in Egypt, and a lot of countries in Europe used some of those suits, and still do. But the French chose differently.
Traditional French playing cards still have these four suits – pike heads, hearts, clover, and paving tiles. And as you read these, you can see how close they are to the suits in the international deck of cards that is in use nowadays. Pike heads became spades in the British/American deck. Spades coming from the Spanish word spado meaning sword, so similar to pikes. Hearts remained hearts, so no explanation needed. The design of the clover has been kept for the suit we now know as clubs. But the name clubs refers to the suit bastoni or bastos still used in Spanish and Italian decks, which actually still depicts a club. So clubs is a hybrid of French and Italian/Spanish. The last of the four suits is the hardest to explain. Paving tiles were indeed rhombus shaped in those days, unlike nowadays when they tend to be square or rectangle. And nowadays, because of the playing card that came to be known as diamonds, we associate a diamond shape with a rhombus too. But we still don’t know why the card known as paving tile came to be known as a diamond. Because hundreds of years ago there wasn’t that association between the rhombus shape and the diamond precious stone. The stone was 3D, a hugely complex shape. A mystery. If you have an idea why – get in touch!