Sapphire Blue (The Ruby Red Trilogy)

Gwen's life has been a rollercoaster since she discovered she was the Ruby, the final member of the secret time-traveling Circle of Twelve. In between searching through history for the other time-travelers and asking for a bit of their blood (gross!), she's been trying to figure out what all the mysteries and prophecies surrounding the Circle really mean.

At least Gwen has plenty of help. Her best friend Lesley follows every lead diligently on the Internet. James the ghost teaches Gwen how to fit in at an eighteenth century party. And Xemerius, the gargoyle demon who has been following Gwen since he caught her kissing Gideon in a church, offers advice on everything. Oh, yes. And of course there is Gideon, the Diamond. One minute he's very warm indeed; the next he's freezing cold. Gwen's not sure what's going on there, but she's pretty much destined to find out.