They could spawn with a set number of stones in their inventory, but all of random types, and that would form part of their loot table, as guaranteed drops. Say upto three different types of stone, up to 20 stones in total. That's all the ammo they have. So it gives you a chance of getting rare stones like Tuff, Scorcia, Phyllite, or Marble; and it'd be a way of distributing other important stone types like Halite, Bauxite, and Limestone. Some seeds require you to travel 50,000+ blocks before finding the one stone type you need. This would make it possible to drip-feed content to players, in an immersive, and fair way.