My very first playthrough I had a slab drop off the crystal lizard in the burg outside the Taurus fog gate. I didn't even know what it was or how rare it was, and ended up wasting it on some random piece of armor. I've never had a slab drop since from any lizard.
That is crazy. Do you kill all of them in the great hollow? I am 90+% sure that it is an automatic drop.
....unless I am the luckiest player the game has ever seen. Because I go kill them on EVERY playthrough, (dozens by now), just to "get the slab". It has never not dropped for me.