[Nerfix]

You know, they could be given a gem-producing non-combat unit ala Ark of the Covenant that has gem production equivalent to a site, but can be killed (even if that wouldn't be easy...) but is useless in combat. Or perhaps not useless, perhaps it could have some Holy magic, perhaps even Holy 3.

Now that would certainly be interesting!

EDIT: For clarification, they would start with the unit but they couldn't build any more of it. Lose it and...well, you're in trouble. Or at leasy you've lost a significant asset.