This medallion looks like a small (3") astrolabe but without any moving parts. The surface is scribed with concentric circles and spirals with small gems of various kinds, each small but perfect, at the intersection points. There are nine such gems on one side and seven on the other.
When found, the item will have between 0 and 15 charges (2d8-1), and can be recharged up to a maximum of 45.
When worn during spellcasting, the casting time of any spell is halved, costing one charge per spell level.
Limitations and Other Features
The medallion has no effect on spells which have no somatic component.
If there are not enough charges for the spell being cast, then the medallion has no effect.
The power of the medallion is not voluntary, so long as it is worn it will take effect and use charges if possible.
The device becomes inert once all charges are gone, although it will detect for alteration magic. It may be recharged by placing the inert medallion into "recharge mode" - for magic users this involves touching the nine gems in a particular order; for others (including illusionists) the seven gems are used instead. What the order is may be determined using divination, finding the notes of a previous owner, or perhaps by consultation with a sage specializing in magic items.
Once in recharge mode, any spells cast while wearing the medallion add one charge per level [edit: instead of having their normal effect]. However, only one spell per level may be used for charging purposes (i.e., to fully charge the item to 45 levels requires one spell of each level from 1 to 9 to be cast while wearing it). Once the user has finished charging the item, a simple touching of the gems in order again will change it back to its functional mode, where it will remain so long as it has charges, regardless of any attempt to use the gems.