Magestone

Mages have an item known as a magestone that they use to help focus their power. Without it they would be subjected to far too much energy and it could cause irreparable damage to the caster. Casting a basic spell without your stone wouldn’t be terribly dangerous, but anything requiring a moderate amount of power is dangerous at best and outright fatal at worst.   Part of a mage’s training is spent learning what kind of stone they can work with. Some are lucky enough to be able to work with less expensive stones but others can only work with ones that are more valuable and harder to find. Size of the stone is also an important factor. Some mages need a larger stone to work with while others can work just fine with a stone small enough to fit on a ring.
Studies have shown that it has nothing to do with how powerful a mage is as there are some weak mages with outrageously rare magestones.
Anyone dealing in magestones must be vetted by the Order of the Rose and submit to regular inspections of their merchandise. Stones sold for use by mages must be as near to flawless as is possible. Too many flaws and the stone will shatter if too much magic is focused through it. Some stones are virtually impossible to find flawless so as close to perfect as possible is the standard. Truly flawless stones are prized and sought after by every mage on Melphor.
Spells themselves are a simple matter of utilizing the correct incantation while focusing your will through your magestone.
Mages can join forces to increase their power and cast more complex spells.
Power is a matter of personal aptitude, much like intellect.