Consider the following situation: The group kills a monster, and one PC wants to use Animate Dead to turn it into an skeleton under his control. But the players have no idea what the monster is, much less how many HD it has.
The PC makes a guess at the monster's HD, chooses an adequate black onyx, and casts the spell. But the guess is wrong, the monster has more HD than the gem is worth. What happens?
Can he even start casting the spell without the right component? If the casting starts but fails and the spell slot is used, is the black onyx burned-out and turned worthless as if the spell was successfull?
Best Answer
A DM ruling is the only answer we have
As far as the official rules go, no one knows; they never said.
Here is literally everything the game has to offer on the subject of how material components actually work:
(Player’s Handbook, pg. 174)
(I just love that “just like a material component” in the XP entry that introduces a new fact not otherwise established about material components.)
You could argue that attempting to cast the spell without the right component means the spell fails, and then argue that—per the line in the XP component section—the insufficient material component is still “expended” even though the casting failed.
But there are a lot of problems with that analysis. For one, “to cast it,” you must have the material components. Without them, you can’t cast it—not “it fails,” but “you don’t cast it at all.” How that works narratively isn’t described at all, but it’s tough to say you cast it—and it just failed—when one of the few rules we have is that you can’t cast it. For what it’s worth, the “Rules of the Game: Magical Oddities (Part Three)” article by Skip Williams (an author of the 3.5e PHB) doesn’t list lacking material components as one of the things that can cause a spell to fail. Furthermore, even if you did rule that the spell fails, it failed because of the lack of an onyx of sufficient value—the onyx of insufficient value wasn’t a (valid) material component for the spell. It’s not at all clear that it would be “annihilated” when it isn’t actually a material component at all.
@Peter Cordes points out, though, that you could argue that you do cast animate dead—it just isn’t powerful enough to target what you want, and having no valid targets, fails for that reason. That’s a plausible rationale for losing both the spell slot and the onyx.
But we can’t really definitively come down on one side or the other. The FAQ doesn’t talk about material components at all. The Rules of the Game articles do, but don’t address this question (I checked). Rules Compendium just repeats the stuff from the Player’s Handbook.
So the DM must make a ruling. The authors apparently never conceived of this as a concern, or just assumed that a spellcaster, knowing how their own spells work, would simply always know what material components they need. For me, I’d just say nothing happens—not even a lost spell slot—since you don’t have everything you need to even try to cast a spell. Peter Cordes has a pretty good argument for ruling the opposite way. Either of those might be consistent with the rules we’ve got. But then, “consistent with the rules we’ve got” when those rules clearly don’t contemplate this situation isn’t worth much, so a DM ruling is really the only way to go.