Okay, here's a theory. The jewel that Rarity gave to Spike in "Just for Sidekicks," while at first appearing practically worthless, turned out to be worth a lot LOT more. Of course, we jump to the conclusion that, by default, the other jewels that Spike has received, or any jewels seen before in the show, must stand at an obscenely over-inflated net worth. Naturally, this assumption can be drawn from the idea that there is a direct correlation between the breadth of an object and the worth of that object.
HOWEVER, it is important to remember that this fact does not stand to be true in all cases. If we are to re-examine Rarity and her repres