First off, this is _not_ a joke. We used to grow these things when I was a wee linoleum lizard in Pennsylvania.
The following is the recipe for "growing" a coal garden:
Piece of coal. (I think we always used anthracite--but that may've been just a regional thing. Bituminous may work just as well.)
1 cup water.
6 tablespoons salt.
4 teaspoons ammonia.
A few drops of food coloring (any color.)
I had actually lost this recipe for over twenty years. (!!) I found it again when I was packing my stuff up for the move to Al-ass-ka.