Hm imo you can't have enough gems
.
With deathgems it is extremely easy cause you get good death casters with a lot of other magics :
Nature : summon lamia queen
Blood : Vampire lord
Astral : Ether gate
With deathboosters you can then summon a demilich and with 3 boosters the demilich can even summon tartarians .
Air is at least of use for the AQs , for boots of flying , SR items and staff of storms for every nation .
Water = Clams
and boots of quickness , maybe waterqueens .
Astral you need a lot for items : Starshine skullcap , lucky coin , Amulet of Antimagic , luck pendant etc. for SCs , Ring of sorcery / wizardy for path boosting .
Lategame maybe wish .
Nature i find especially lamia queens useful and there is GoH . If you setup a wish/tartarian factory you need as well GoR .
Earth = Dwarfen hammers etc.
. Good summons for special purposes . Forge of Ancients
.
Fire : lategame flames from the sky , maybe lightless lanters for research speed up , gold , fever fetishes , flamebeaus . Imo the gems with the fewest uses .
Blood you can never have enough
.
Imo good site searching , no matter if by accashic record , rainbow mage or single path search spells pays always off in the medium/long run .
Death , Nature , Earth and Water are imo the most important gems .