[Kobal2]

It means that any mage casting an Alteration ritual in that province will save 20% of their gems. So, if e.g. that mage is casting Wish he'll only use 80 pearls instead of 100.
Which breaks the game quite thoroughly, I might add.

There are similar sites for most other schools of magic, and each of them makes their owner significantly more powerful. They really are the jackpot of sites.

Not sure if the bonus also applies to combat spells that use gems (such as, say, Curse of Stones for Alteration) cast in the province.