Spike finds a strange gem while helping Rarity hunt for gems, and senses that this is something that he needs to keep for himself, maybe even ask Twilight is she can find out what is making this gem feel so different than any other gem he's held before. He hides the gem under his tongue, not letting Rarity know that he had found it. Spike soon finds that finding the strange gem was one of the best things to ever come his way, doing anything he can to protect his secret and savor every moment that he got with it.