In English, rhombus (◊) is often called a diamond. But why? The earliest and simplest of diamond cuts, the point cut, is a regular octahedron. A projection of an octahedron which looks like a rhombus could have been a reason to name this shape a “diamond”. Somehow this geometrical observation did not find its way to other languages. By the mid-15th century, more advanced cuts were introduced and “diamond-shaped” diamonds were passé.
The suit ♦ of a French deck, known as carreaux (tiles) was derived in 15th century from the German suit of Schellen (bells). Those round bells look nothing like the tiles, though. One might think that French, for simplicity sake, just decided to “square the circle”. According to Wikipedia, however, “between the transition from the suit of bells to tiles there was a suit of crescents”. Interestingly, the bell—diamond connection of ♦ still lives in the Russian name бубны (or буби), from бубенцы, “bells”.