You can channel extra energy into this aquamarine crystal to create a sustaining power for your spells. When you cast a spell of 3rd level or lower that has a duration of 1 minute or longer, you can touch this crystal and spend additional spell slots to extend the duration. You can pre-charge the crystal up to halfway prior to the casting (13 charges).

 This spell can hold a maximum of 27 charges. You may expend spell slots to charge the crystal using the conversions: (level-points) (1-2), (2-3), (3-5), (4-6), (5-7), (6-9), (7-10), (8-11), (9-13). 

Using the conversions in reverse, the duration of the spell is multiplied by the number of times it can be cast using the crystal's charges. For each charge that isn't divided evenly, the spell extends by half the original duration. When you first cast the spell, you must expend an additional spell slot of the spell's level to link the crystal to the spell effect. Spells cast at higher levels than their original can benefit from their increase but count as the level they were cast at for the purposes of their duration. 
EX: The crystal has 13 charges and then you cast charm person (1st level, 1 hour) at second level, affecting 2 targets. The spell's duration now extends to 4 hours, 30 minutes, and effects both targets. 

High-burn: When you link the crystal to a spell, you can set the duration to burn at 20 charges per hour. While this effect in ongoing, the crystal burns with an ethereal blue fire, which burns to touch. For as long as this occurs, the crystal cannot be handled in any way, this includes movement, charging, or relinking it. This has a 10% chance of breaking the crystal.

This crystal must be within the range of the spell cast in regard to the spell's effects. If the spell is concentration, the caster will need to hold it and remain in the spell's range. Additional crystals can be linked without additional spell slots and add to the duration. 

Notes: a spell caster