Synthetic Crystals
Bismuth crystals are laboratory made, not naturally occurring. The natural form of bismuth, a metal and element, shares the property of being denser in its liquid form than its solid form with the only other elements like this which includes silicon, gallium, germanium and plutonium. This is because these elements don’t form H-H bonds under standard pressure. It’s also diamagnetic, as in, it is repelled by magnetic fields. It’s solid form is more massive or a flat hexagonal grouping.
It has lots of cool characteristics. But the rainbow is not natural at all and neither is the shape people associate with bismuth. The only ‘rainbow rock’ out there is known as Bornite, also called the Peacock Rock because it tarnishes in blue and purple hues. But opals can be so fantastic in appearance with colors that they have their own descriptor for appearance, opalescence, which can also appear like a rainbow.
Ultimately, there are lots of minerals out there, especially ones with impurities, that really stand out in proud rainbow fashion. But the title and naturally occurring ‘rainbow rock’ is Bornite.
https://geology.com/minerals/bornite.shtml#:~:text=Bornite%20is%20a%20copper%20iron,of%20many%20sulfide%20mineral%20deposits.
. https://www.minerals.net/mineral/bismuth.aspx
https://en.wikipedia.org/wiki/Opalescence