You can, but Empowerment is in general a waste of gems. Water is an easy skill to increase. Robe of the Sea is (Water 3) and Water Bracelets (Water 1) are easy forging items to get higher up on the food chain.

Water also has the very useful Voice of Tiamat spell (if you have a decent number of water provinces on the map)

A Rainbow is one who has many magical paths, but not always "all". Fire and Earth are common, because you can Alchemize their gems, while Air has great items to forge. Astral is always good, and Nature has quite a few good items as well.

Rainbows are used for gem searching moreso than spell casting. Most spells they are regulated to are summons that summon a more focused unit that can stay in it's speciality element.