Wanted to say "as available", as an easy to forge anti sacred rush counter most nations may use (increasing gem cost would also increase path requirement, so make it an item for high nature nations/pretenders only).
What I first registered on the forum, a thread exactly the same as this one was going around. It's definitely a good shield, although probably not overpowered. The more a player relies on a few powerful units, the more Eye Shields hurt. But it's really more of an annoyance than the kind of thing that can turn the tide of war.
Mildly related question: If a unit loses and eye and you give them an artificial forged eye, do they put it in the empty socket or do they poke their other eye out?