Currently this is exactly the same as Potion or Elixir Mastery. When you do a transmute, you get a chance to proc more than one. So if you're trying to transmute Primal Might from...whatever, you might proc up to four or five Primal Mights in one shot. Just like potions.
There is currently some debate as to the usefulness of this. If it doesn't proc, you're stone is still on cooldown for the duration of that specific transmute (24-48 hours for most if not all known transmutes). The debate is that, since you are limited only by the amount of raw materials you possess with the other two mastery's, the cooldown on the Alchemist's Stone makes this specialization rather prohibitive.