Aura strong (no school); CL 11th
Metamagic gems can be used to enhance spells with various metamagic effects, similar to the way metamagic rods are used, save that metamagic gems are consumed when they are used. To apply the effects of a metamagic gem to a spell, you need only use the metamagic gem as an additional material component for the spell in question.
When you apply a metamagic effect to a spell using a metamagic gem, the spell slot of the altered spell does not change. You may only use one metamagic gem on any given spell, and while that spell cannot be further enhanced by a metamagic rod, it can be used with spells already enhanced in the normal way via any metamagic feats you possess. In this case, only the feats possessed by you adjust the spell slot of the spell being cast.
Use of a metamagic gem does not confer the associated feat on the owner, only the one-time ability to use the given feat. A sorcerer or other spontaneous spellcaster must still take a full-round action when using a metamagic gem, just as if using a metamagic feat he possesses (except for quickening diamonds, which can be used as a swift action).
The metamagic gems presented here are not the only such gems that exist. Other metamagic gems based on metamagic feats from other sources could easily be created—their costs should follow the pattern presented here (1,000 gp for gems that alter a spell slot by 1 level, 2,000 gp for those that alter spell slots by 2 levels, and so on), and each new gem should have a corresponding unique gemstone type associated with it.
Craft Wondrous Item, appropriate metamagic feat; Cost 1,000 gp (empowering topaz), 500 gp (enlarging amethyst), 500 gp (extending garnet), 2,000 gp (maximizing sapphire), 4,000 gp (quickening diamond), 500 gp (silent spinel), 500 gp (still amber), 2,000 gp (widening emerald)