Since when do dragons eat rocks in the first place? I thought mostly ate princesses.
Spike eats diamonds in My little pony. I think that is where that meme comes from and I've seen a few "dragon-kin" posts where people talk about their "need" to eat jewels and diamonds in particular.
But doesn't Spike eat other things as well? If I remember correctly, jewels seem to be like candy for him, not a staple of his diet. Why would someone feel someone feel some overpowering need to eat something that's not essential to their species? Oh, right, because they're a bunch of bored, embarrassing twelve year olds.
The show also has dragons making hoards of gems and eating them. I suppose it is possible that an adult dragon would build a food storage with nothing but the equivalent of candy but unless there are scenes showing a dragon other than Spike eating something that isn't rare gems I have another theory.
Maybe it's like with owners who force vegan diet on their cats? The rest of the food that Spike eats is really unhealthy for him but he learned to eat it because Twilight either can't afford to get him proper food or because she has been using up Spike's food allowance to buy more books for herself. After all, Spike grew up with Twilight and it has been shown that the pony knowledge of dragons is limited and even Spike himself doesn't know how to live as a dragon.
(Note that I have not seen all the episodes so there is a chance that my theory is flawed.)