Artificially grown bismuth crystals are arguably magnificent, as seen in the image below:
Image source: Minerals.net
The website states that:
They have hopper-like growths in pseudocubic crystals and are usually coated with chemicals to prevent tarnish, thus maintaining the silver-white color. Sometimes the coating gives a colorful effect on the bismuth.
What coating gives the colourful iridescence? and what chemical processes cause the colourful iridescence to occur?