Casting even small spells would require a fair bit of practice in quiet areas; most likely, given Rita's highly impatient nature, the best she'll be able to do until she gets a memory or two of how to work it is get the gem to light up.

While Ancient Catastrophe (which will be way too complex to apply until she's mastered several lesser spells) is basically a highly focused area explosion, the first couple spells she's likely to actually figure out are Fireball (a set of burning orbs, not an explosive ala D&D) and Splash (a small-area-effect water rush spell). I do need to take time to figure out some of the utility magic she'd know, as well.