Sand Casting Skulls
If you have access to a sandbox, or even just have some sand and a box you can put it to good use in making some skulls using the sand casting tutorial that partsman at HalloweenForum wrote. You’ll need some play sand, a box, some great stuff, and a foam skull (or whatever you chose to cast).
Once you’ve got all your supplies, you’re basically just pressing the skull into the sand and packing the sand really tight around it. Once it’s packed, just remove the form and add some great stuff to the impression you’ve created. As you can see the resulting product has a really great looking stone appearance to it.