Many rocks in deserts are covered with an unusual material known as “desert varnish”. What’s most interesting about it, is that ancient people carved images onto it that can still be found today. Even though scientists managed to find out what it was made of — clay, iron, microorganisms, and manganese — it is still not known what made the varnish in the first place.
The main theory is that the varnish was produced by microorganisms found in it to protect themselves. But this is only a theory.