Here's why:

* Crystalline structure: Bismuth forms crystals with a unique, layered structure.

* Light reflection: The way light reflects off these layers creates interference patterns, similar to how a soap bubble or oil slick appears colorful.

* Oxidation: A thin layer of oxide forms on the surface, contributing to the iridescent effect.

While bismuth is often described as having a "rainbow" color, it's more accurate to say it displays a range of vibrant hues depending on the angle of light and the oxidation of the surface.