Crystals

Magical essence can be stored in crystals. Storing your magic in a crystal will cause you to become physically drained and you will need to rest before your magical strength replenishes. You are only able to drain your magic in small amounts, as the spell itself is very draining. 
A magic user will be able to sense their own magic by touching the crystal, but otherwise only those with the special ability to sense magic would be able to tell the difference between an empty crystal and a full one. 
Breaking open a storage crystal will release the magic and can allow someone not born with magical abilities to cast a spell. 
A magic user would be able to draw the magic back out of a crystal, without breaking it, even if they are magically exhausted, whether it was originally their magic or not.
The magic will appear as a shimmer in the color of the aura of the person whose magic it is and visible aspects of the spell will also be in this color.
In the case of crystals, size does matter. The bigger the crystal, the more magic it can store. 
Pebble - a pebble sized crystal will store just enough magic for one spell up to level 20.
Rock - a rock sized crystal will store enough magic for five to ten spells up to level 20.
Boulder - a boulder sized crystal could hold enough magic for twenty to forty spells up to level 20.
Mountain - 50+.
Though a crystal stores enough magic for at least one 20 level spell, if your character is a non magic user or inexperienced, then it does not guarantee their ability to successfully cast a 20 level spell. The magic has the strength for it, but if they lack the skill to harness and control that magic, the spell will very likely cast at a lower ability score. In these cases, take an average; if your character has zero magical experience and they have a magical crystal that has enough to cast one 20 level spell, they would most likely cast a spell at a spell casting level of 10. Likewise, if they are a magic user with a level 10 casting ability, a full crystal would yield enough raw power for their spell to have the casting effect of a level 15 spell caster.
See also: Magical Stats