[chris (OP)]

There's a rather nice question been added to another page on the site by "Immi" who says:

"What did I find seeping out of a wet, iron and clay rich cliff? It was definitely a film, with a metallic rainbow sheen...."

It was on this page: https://www.thenakedscientists.com/articles/questions/why-does-petrol-create-rainbow-water

I think what is being referred to is the film that resembles the rainbow pattern that oil makes on water and which you sometimes see leaching out of the base of cliffs.

I have actually seen this effect myself, and I presume it's because something organic is rotting away inside the cliff, or percolating through the cliff, but can anyone with more geological knowledge please shed some light on this for us?

[Colin2B]

It's an interference pattern.
The slick on the surface could be organic or just contain some oil. Light passes through the thin surface layer and is reflected from the top surface and also from the water/oil interface and if the top layer is thin enough there will be constructive and destructive interference of the reflected light frequencies. Because the thickness of the top layer varies slightly, typically it often thins towards the edge, different frequencies - colours - will appear as bands where there is constructive interference and absence of that colour where there is destructive interference.
Would be interesting to know what the surface film is.

[chris (OP)]

Thanks Colin; but I think what the person actually wants to know is what the stuff "is" that is making the oily discolouration on the water and rock surfaces. Anyone know?