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?

Update

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.

There is also this article about The Flight of Dragons (1982) that says: