[Spoilers Vol 4, Ep 1]

Three part theory:

- Summer Rose is missing her left eye.

- Summer lost her eye when she gained Maiden powers.

- Ruby is going to lose her left eye by the end of the volume but will be mostly okay with it.

Justification:

- The only image we have of Summer has her cloak up in a manner that actively prevents us from seeing the left side of her face.  While we can't say a lack of showing it is proof it's gone, we also can't say for sure she has one.

- Something awful happened to Cinder when she crossed paths with Ruby.  Sure the dragon got frozen, but Cinder got physically torn apart and even lost her left eye.  However, Salem points out directly that there is some sort of adverse effect when a silver eyed warrior meets a Maiden.  If we follow the full crack theory that Summer was a Maiden, it seems unlikely that a silver eyed warrior would be able to take on the Maiden powers with out some sort of sacrifice.  

- Cinder wants revenge on Ruby, and Salem and team WTCH seem happy to help provide it ("An eye for an eye!").  Since Cinder lost her left eye, it stands to reason if Ruby is actually captured and tortured they'll start with her left as well.  While the loss of depth perception will definitely suck, Ruby demonstrates being cross-eyed dominant whenever she is using her sniper rifle (i.e., though she is left handed, she is right eye dominant), so at least that aspect of her fighting won't be impacted.  Besides... "It's like they say, 'Like mother, like daughter'."