NeoPhantom
Skeletron Prime
Actually if we could take that and add in some terrain effects to make a Geomancer class I would never play anything else ever.
That seems hard but...
It might be somewhat doable? For example, having the special summon a much larger gem which provides a boost depending on the gem consumed and the level of the player to all players nearby (similar to the campfire and similar structures).
As for strenghtening the player, having the Large Gems could modify the damage values and add aditional effects to gem staves depending on level. This would make the class supportive with a chance to defend itself.
I dont see Disgaea's Geo Effects being in play in Terraria in the same way as they are in that game...