[Yrkoon]

As well as empowering your mages (permanent increase for a lot of gems) you can also use the gems in combat to boost your level temporarily. You can use one gem per spell to increase you level by one. It's a good way of casting spells you would normally not be able too.

For example a weak fire mage with one level in fire magic can use one gem to cast phoenix power (level 2 fire spell). Phoenix power boost your level in fire by 1 for the whole battle. You mage is now level 2. By using another gem, your mage can cast a level 3 spell. All of a sudden your crappy mage is quite deadly.