It seems to make a certain amount of sense to me. Not only do their bodies appear to be faceted, similar to quartz and other silicate crystals, but practically everything they eat seems to be "crystal _____", which could be because their digestive systems are able to handle crystals only. Organic crystals are largely things like hardened sap or fossilized animal remains, which don't quite evoke the crystalline theme as well as silicon-based ones do, and silicon is considered a potential crucial element for hypothetical extraterrestrial life forms. Not to forget that silicon and its compounds are highly abundant, so it seems likely a creature would be adapted to thrive on the stuff.

I came across a similar concept in the Discworld books, in which trolls are silicon-based beings who consume rocks and acids as part of their normal diet, among other quirks. Basically, how feasible is this idea? I am considering it for a future fic, but I mainly ask out of general curiosity. Penny for your thoughts?

Been there, done it, the idea pans out just fine. Also, dragons make for excellent sulfur based lifeforms.

5635347 I can see this being feasible if magic is involved, which it almost certainly is. Without magic, one problem with animals made of crystal is the fact that crystals tend to be pretty rigid. Limbs could have joints in them, of course, but what are you going to use in place of muscle? Proteins come in the form of linear chains, which allows them to coil or fold and contract. A crystal, by definition, is a three dimensional lattice of particles arranged in a regular pattern. The atoms or molecules in a crystal tend to be about as tightly packed as you could possibly get them. Pizoelectric materials exist that will contract if a voltage is applied to them, but this effect is tiny.

It's possible, silicon can behave similarly to carbon so it works on that level. There'd be some interesting electrical effects so you could have fun with that.
I'm not sure eating crystals would actually give them energy. I'm not a biologist and I'm not even sure how it works for carbon based life.
The big thing is how closely related are ponies and crystal ponies? If one's carbon and the other's silicon the answer is not even by a long shot. It's a massive huge coincidence. I'm not even sure they'd be in the same kingdom, or possibly even domain (again i'm not a biologist).

That's an excellent idea - except that the show's been inconsistent in showing ponies as crystalline or normal flesh-based. For a while at the end of S3E2, the Mane Six were crystal; for most of S6E1, we didn't see a single crystal pony in the Crystal Empire. This'd be a great idea, but you'd need to write this off as inaccuracies of the camera.

While silicon can form complex chains like carbon, its larger size means that silicon-based life would have to exist at much higher temperatures unlikely to be compatible with co-existence with carbon-based life. Isaac Asimov (who, in addition to being one of the most famous science fiction authors, also has a PhD in biochemistry) has a nice essay on alternate chemistries for life: http://www.bigear.org/CSMO/HTML/CS09/cs09p05.htm