Tools:Patterns:Make Fractal Pattern
to create a fractal pattern, any size will do but I suggest setting the sliders to
Next, choose the Gold 3 gradient from the Colours floating window and apply it to the image by selecting Tools:Gradations:Express In Image
, giving you something like
Not bad, eh? To make it look as if acid had been eating away at the surface, use Effects:Surface Control:Apply Surface Texture
, selecting
It's worth mentioning here that as it was created as a fractal pattern, you can tile it seamlessly.