Is a build-up of undisturbed bacteria the cause?

[MattW]

Just been moving some lava rock and found a decent majority of these rocks have yellow/brown patches. These were all located at the bottom of the rock (in contact with the substrate). I'm assuming one of the many anaerobic bacteria's is the cause?

The smell is lovely too!

 

[Jinx_]

Sorry the cause is me. I don't shower for weeks