[Panpiper]

If you want the acquisition of certain unit types to be somewhat dependent on comprehensive site searching, might I suggest making those unit types summonable, 'but' for gem costs require an equal number of 'all' gems. Given a race with three or four paths of magic normally available to it's mages, it is quite difficult to wind up with anything close to a balanced number of gem production sites. It's quite common to be generating 15 or 20 of one's nations path types and one or two of the gem types not in one's nation's paths. Even if the elves have mages that when combined to search in all paths, simple chance is going to make some gem types less common for that player's game than other types. If the 'wished to be rare' summonables require a bit of 'all' gems, they will be limited to the rarest of the gems that have been found thus far.

Edit: Ok, I guess I was talking out of my hat. I cannot see any way now that I've actually looked at it to implement what I said. It would appear it is not possible to create ritual spells that use more than one gem type.