Freya and the magic jewel / Joan Holub and Suzanne Williams.

Enhanced descriptions from Syndetics:

From the authors of the Goddess Girls series comes the first book in a brand-new series steeped in Norse mythology, magic, adventure, and friendship.

"Gold and Gullveig I cannot see. But here is a vision that comes to me; Adventure for you is about to start. Near Asgard you must find the heart. A Secret world there hides away, which holds the power to stop doomsday!"

When eleven-year-old Freya hears that prophecy from her magical jewel, she isn't sure what to make of it. Doomsday? She will find the key? Mere seconds after that prediction, she receives a mysterious invitation to Asgaard Academy from the powerful Odin, who commands her to "bring her magic" to Asgard.

With encouragement from her twin, Frey, Freya reluctantly heads out on their new adventure. And Freya's first challenge begins before she even steps foot in Asgard. While trying to navigate the treacherous BiFrost Bridge, she drops her magical jewel off the bridge, and a sneaky pair of dwarves take her jewel down to the world of Midgard!

Without that jewel, Freya thinks she is powerless. But with the help of her pod-mates at Asgaard, Freya discovers a world that is bigger and more mysterious that she ever imagined! There, she learns the true terror that Ragnarok--the doomsday her jewel warned her about--could mean for Asgard Academy if she and her new friends, the Thunder Girls, don't stop it!