Heat resistant casts

That would be great for custom figure parts and accessories and I might use it in future but what I am trying to make at the moment is a actually a piece of jewelry itself. I'm working on a metal ring.

I checked out a book from the library on jewelry making one time, and the author had a metal-casting technique he used with a cuttle bone (those things that pet birds use to sharpen their beaks). Basically, you cut the cuttle bone in half, carve out the object you want to cast (I believe he was doing a cross or a sword), seal it back together, and then pour your liquid metal in. The cuttle bone gets charred something fierce, so it's only good for one go, but it works. Obviously this will only work if you're making something relatively small and thin, but for stuff like toy swords or axes, it'd be ideal and pretty cheap.

How about simple dirt? You could bury your wax sculpt in tightly packed dirt and pour the liquid metal into that. I saw a scientist do that on television with an (empty) anthill (he wanted to make a model of the ant's tunnels and caverns, which ended up looking like an inverted tree, it was pretty cool). I think he used brass for the metal, but I don't remember for sure. I suppose you'd probably get some of the dirt's gritty texture on the surface of your cast, but, depending on what you're making, that might actually be desirable (I.e., pitted metal, textured monster skin, etc.)