A rhombus is just an equilateral parallelogram, and parallelograms are the easiest way to approach vector cross products. And you're 3/4 of the way there when you do tip-to-tail vector addition. They should have come up at least a couple times in geometry. Rhombi pop up as faces in lots of polyhedra, and tiles in lattices. I don't know much about crystallography, but I'm sure they make an appearance.