Chad Rose asked me for details about catalysts on G+ and I answered in some detail there. In other words, he asked about what catalysts are. The short answer is: it is up to the DM. The long answer, if it is of interest to anyone, consists of a few various ideas.
One way of physically defining a catalyst is to imagine a cloth bag filled with a coarsely ground powder. To make an elixir, the hewcaster spreads the powder on a stone and says a few incantations. Over the course of ten minutes, the stone will appear to become roiling liquid while maintaining its shape. Near the end, a small piece, a bit smaller than a chicken egg, will separate from the larger stone. Both pieces will stop moving. All of the catalyst powder will be on the larger piece of stone.
The powder can then be poured back into the bag from the stone. Absolutely no powder is lost by making an elixir. The only way the hewcaster loses powder is if he or she spills some on the ground.
Thinking of catalysts this way make an elixir less of a potion and more of a lozenge. The imbiber puts the elixir in their mouth and it quickly dissolves into a liquid and consumed. As a matter of convenience, I assume the stones are not easily broken unless the hewcaster hurls them at a target.
As an aside, if may be worth it to allow a hewcaster the ability to use a sling. It can provide some range without giving him or her any other advantage. If a hewcaster throws an elixir at a target, he or she must make an attack roll anyway...
Another way of defining the catalyst is to imagine it as a short length of cord or rope. Once wrapped around the hewstone, the spell caster puts on a pair of gloves to safely handle it. Picking it up, he or she tilts one corner of the stone toward a container and liquid drips from the stone into the container.
This makes elixirs just like potions, but requires the hewcaster to carry around a bunch of empties. This seems prohibitive at higher levels or extended campaigns. Still, the DM can hand wave the whole thing. Who knows, maybe the hewcaster was a glass blower before pursuing magic.
If the DM would like the used hewstones to have some kind of mark to distinguish them from unused ones, run a variation of the first idea. In this scenario, the catalyst is a triangular piece of rock with a thick groove cut out of the long edge. The catalyst can slide over the corner of the hewstone and says a few choice incantation. After about 10 minutes, the catalyst is removed taking a piece of the hewstone with it. That fragment of the hewstone falls out of the catalyst and becomes an elixir.
With this method, the hewcaster can now more easily go through a stockpile of stones to determine which hewstones can be used to make elixirs and which ones need to go back to the lab for spell research purposes.
To really throw a wrench at your players, make all three processes work so that there is a chance that a found hewstone may or may not work to make an elixir.