In your example the level-1 fire mage can eventually cast Summon Lesser Fire Elem but with 3 gems.
He should start by casting Phoenix Power (by using 1 gem to boost his level to 2), and then the Summon, again needing 1 boost and the gem for the spell (he can now spend a total of 2 gems as he has become level 2).
Not very useful for this spell, but for example it can be a deal to cast Fire Arrows this way in case of need...
