Fother J: Excellent work. Have you checked whether it looks OK without an outline, though? (Otherwise it would be great if you could add one.)
Jumpy: Your #4 is definitely going in the right direction, except for three things:
- It's too big, though I like the shape (32x32 is the maximum icon size; the size of your image implies that the lump is huge).
- The right-hand side is too bright and has too strong a gradient
- The left-hand side is shaded too evenly
Your gradient for the top partition implies a dent, that one is fine. Your gradient for the right-hand side, though, starts out at the maximum brightness-- but the right-hand side doesn't face the light directly at all, so it should be darker overall.
I've marked up the left-hand side in three colours. The red and blue areas are only slightly towards the direction of the light, so they should be a darker shade of grey than the green area, which is facing almost directly towards the light.
Of course, your clay has a very rough surface, which means that it gets a lot of ambient light (hence the contrast will not be greater than what you have there right now.)