Sea Glass by Maria V. Snyder

Transformed from a timid girl who could make magical glass to a magician who can readily steal the power of other magicians, Opal Cowan is considered too powerful and possibly a threat to Sitia. Though ordered home by the Council, Opal is no longer content to follow them blindly and instead goes off on her own to search for Ulrick who she believes to be a victim of corrupting blood magic. But Opal’s plans hardly go as she hopes, and soon she finds herself embroiled in another deadly plot. Word of Opal’s newfound abilities has spread, and now there are people after her for their own selfish gain. With numerous enemies everywhere she goes, Opal no longer knows who to trust. All she knows for sure is that there are more lives than just her own at stake and that she can’t solve this mystery on her own. 

Opal Cowen has been summoned back to Sita's Magicians Keep by the high council. Never a good sign. After an unsuccessful trip to find her lost friend Ulrick who has switched souls with her long time enemy, Opal heeds the summons and returns only to be put under house arrest. Her new found power of siphoning the magic of others has the Keep on high alert and she must earn back their confidence.

At the same time, Opal is determined to prove that Ulrick is no longer himself, and becomes entangled in a plot to steal control over the glass messengers Opal creates for Sitia. Unless Opal can figure out who is behind the plot, all of Sitia may be in danger from something much worse than just some stolen magic...