With this rock I noticed how weird and somehow crappy Unwrella (3dsmax plugin for unwrapping) works and it's a shame. It doesn't take the whole shape into account or at least the flatness of some parts of the mesh. You end up with a fancyful UV mapping with flat parts being cut right in the middle. The thing is when I decimate the number of polygons, I use the Balancer which works with the UV map to preserve the seams as much as possible. So if the seams are wrongly placed you end up by losing the overwhole shape of the object. You will lose the sharp edges & corners if they are in the middle of a UV chunk.
So I had to to the unwrapping by hand.