Weird art effects

Hey there :slight_smile:

I have a problem with some weird art effects in my render. I have a non textured stone. That looks pretty normal before rendering, but when I hit render I get some weird black spots. I have no idea what causes them :confused:
I am using cycles with all the default settings. And smooth shading on the object itself.

One more image:

High contrast lighting + low poly object + unbiased render engine = terminator artefact.
Not really another option but to increase the mesh density.

Thanks for your help :slight_smile: