I have a couple of ideas regarding the offensive spells. There could be a "raw" form of magic that either shoots out a blast of energy, or forms a simple magical barrier, but this form would consume the gemstone in the process. Then the elemental form could be more advanced and difficult to execute, but requires some kind of criteria to execute. If we want to include different gemstones later, then maybe each gemstone is in tune with a special element, but to activate that element it requires that the user achieve a certain mental state. Perhaps sapphires could be curiosity, since that seems to be the common trait between our characters. Activating an elemental spell would draw on the caster's strength and preserve the gemstone. If you like this system we could work out the finer details, or we could try a different system if you have any different ideas.