To create the effect of rough-cut stone on the edges (and also to hide the joint between the front and rear sections), apply a generous coat of monster mud. This will also help project the edges from chipping.
What is Monster Mud?
The basic recipe for monster mud is 5 parts drywall joint compound and 1 part latex paint. You do not need to follow these proportions exactly, and can adjust it to make it thicker or thinner as desired. In our case, we added 1-2 parts sand to make it even rougher.
It makes no difference what color paint you use, as you will be painting over it with black in the next step. We usually use leftover paint from other projects, or buy the paint that is mixed at the store, and for whatever reason, is not purchased by the customer (heavily discounted). After we started using it, we summized that it was because of the hideous color that reminds us of dirty diapers!
Apply the mud with a putty knife or brush. We dabbed in on with a brush to give it the texture shown in the photos.
You may choose to paint the rest of the tombstone at this time with the same paint used in the mud. We like to do this to make sure we have edges of the mud bonded to the stone, and gives us a good primer for the black.
After the mud has dried, knock off any points or sharp edges with rough sandpaper.