[wolf_of_benjamin]

...tact with air?

[Arkcon]

Yes. 

http://www.chemicalforums.com/index.php?topic=23002.0

The picture I posted in that years old thread is long gone.  But anyway, you can find them (and their explanation) on Wikipedia: http://en.wikipedia.org/wiki/Bismuth#Physical_characteristics

[wolf_of_benjamin]

I bet it reacts suddenly with diatomic nitrogen; maybe this creates a layer of BiN (Bismuth nitride) which creates as see of electrons around the metal and you get all 7 colors both have 7 electrons on their outer shell.

[DrCMS]

I bet it reacts suddenly with diatomic nitrogen; maybe this creates a layer of BiN (Bismuth nitride)

I bet it doesn't and if you'd bothered to read the link Arkcon posted you'd know the iridescence is due to the formation of an oxide layer.  Quoting from wikipedia - "The variations in the thickness of the oxide layer that forms on the surface of the crystal causes different wavelengths of light to interfere upon reflection, thus displaying a rainbow of colors."