[Corinthian]

I once found a province with an Alt 20%, a conj 20% and Chillsick swamp in it. Fun.

Also, to ad up on what Knai said. Unlike hammers, discount sites seem to round up the cost. So a 6 gem spell cast at a 30% bonus site would cost 5 gems. If it had worked like with a hammer it would have rounded down and costed 4 gems.