[Peter Ebbesen]

To perform a ritual summoning, you must have

• A mage with the required magic skills for the ritual
• Who must be in a province with a laboratory
• And you must have sufficient gems stored in your gem pool (not on the mage - gems in the mage's private box are for use in combat)

Assuming that your mage has the required skill level, it sounds as if you are giving the gems to the mage instead of leaving them in the pool.

As for gameplay effects of high dominion, yes, there are many. Some tied to scales, some to specific magic spells, some tied to morale, some tied directly to your pretender god and to your prophet. One is tempted to suggest that you read the manual if you have the full game and not the demo. But, to put it short, high dominion is with very few rare exceptions better than low dominion.