mosehansen Faerun, again.

[Cohen]

There's an house rule in some games I've joined that doesn't limit the gem producing items, but that you can't move your gems (not slaves) from commanders to lab.
So you can only move gems from commander to commander.
What's now the use of a clam?
Fuel your combat spell in battle without having a supply line of scouts that have to pass gems each other till the frontline. (when you're lacking of money, this is a far better choice than building a lab that could fall in enemy hand ... scouts hides and sneaks, and if you lose your army, you can mostly retreat your gem bringers where a new army is growing.)