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I'm a little confused about blood slaves and blood gems...

There's an item I wanted my mage to create, but required blood gems, so I go on a blood hunt...I find 5 virgins, and thought by sacrificing them I recieved blood gems...I'm playing Marignon, and have 2 mages with 1 in blood magic, but don't see an option to sacrifice...

After looking around this forum some, let me see if I have this right...

Can Marignon mages perform a sacrifice at all, even if they have blood magic attribute?
You can sacrifice slaves in combat for blood magic (think the answer to that is YES)?
Outside of combat, sacrificing only ups your dominion, and then only for certain Races?
Only way to find blood gems is to find a blood site/shrine/mine/whatever?

Anything I left out?

Thanks for the help....