Timebound (The Chronos Files, Bk. 1) by Rysa Walker

Kate is stunned when she learns her grandmother's secret. She is even more stunned when her grandmother gives her the blue medallion, and she learns that she is intricately involved in this secret...

This blend of historical fiction, romance, and science fiction grabbed me early on. The blue medallion is a portal for time travel (something I seem to like!) and Kate's story takes her between past and present as she tries to save her grandmother by unraveling a conspiracy.  Interesting twists keep it unpredictable, and the thread of romance with Trey - the boy she meets during her time travel - help ties the story together as well.