In My Little Pony: Friendship is Magic Spike is a Dragon that eat gemstones. Here's some proof for the distrustful:
Maybe it's me that don't know a lot about the Dragons lore, but it's the first time I've heard of a dragon that does that. While I'm sure there's a lot of minerals in gemstones, but I doubt their caloric values. It's just counter-intuitive to make dragons eat gems, and since everybody seems to accept that without frowning, I wonder from what lore this dietary habit was inspired.
I have scanned the Wikipedia article about Dragon and found nothing in it.
So, what is the inspiration of Dragons eating gemstones in My Little Pony: Friendship is Magic?
While I'm still hoping someone with more knowledge of the dragon Lore will answer this, I will share with you some information I found:
According to Wikipedia, Dungeons & Dragons had some gem dragons dating back to 1980, and several of them are said to eat gems, but I have no idea if their dietary preferences were described back then.
According to the movie, dragons eat gem to crush limestone. limestone being rich in calcium creates hydrogen when in contact with stomach acid. Hydrogen being lighters than air makes the dragon "float" and is also flammable.