The original French name of the suit is Carreau; in German it is known as Karo.
In older German-language accounts of card games, Diamonds are frequently referred to as Eckstein ("cornerstone"). In Switzerland, the suit is still called Egge (=Ecke i.e. "corner") today. The term "Karo" went into the German language in the 18th century from the French carreau, which goes back to the Latin word, quadrum, meaning "square" or "rectangle".
The diamond typically has a diamond shape, a parallelogram with four equal sides, placed on one of its points. The sides are sometimes slightly rounded and the four vertices placed in a square, making the sign look like an astroid.
Normally diamonds are red in colour. They can however be depicted in blue, which is the case for example in bridge (where it is one of the two minor suits along with Clubs).