I was actually planning on having a Terram vis source in Guiverna's quarter, being a tree that bears turquoise instead of fruit. When picked at the peak of ripeness, each gem is a pawn of vis. Picked too early or too late, and the gem simply crumbles to dust in the hand.
Not sure, though, how to reflect this in the building rules. I want to say that it would be Personal Resources, with "Lesser" being three pawns a year and "Greater" being ten pawns, but I'm not sure.