What is the difference between drywall plaster and plaster of Paris?

The reason I ask is I would like to try my hand at carving in plaster, but when I tried to make a block out of Plaster of Paris I couldn't get the center to dry, no matter how long I waited. So, How can I make a block of plaster than can be carved, and is there a difference between drywall plaster and Plaster of Paris? Should I try using drywall plaster?