I read this book a few years ago which was part of a series, but I only found the first book and now I've forgotten the name.
The first book featured 3 children/teens from different places that end up on a quest to search for magic gems/stones together and the story takes place in a medieval kind fantasy world. One is a girl, while the other 2 are boys. One of the boys actually finds a stone/gem while he is on a school field trip (I think the gem/stone is red in colour), and when he picks it up, he gets transported to the fantasy land but ends up in some sort of military training ground/camp somewhere in a desert type area. He isn't familiar with the fantasy world. The other 2 characters each start out on their own on a quest to collect some magical gems/stones which once collected, can be used to fight an evil ruler (I guess). But halfway through the book, the girl and boy meet up and continue the quest together while the third boy(transported one) doesn't know that the stone he picked up is actually one of the magical stones others are looking for.
A few other details I remember:
- The stone the boy finds is red in colour I think
- The cover of the book had a border depicting the stones
- It's part of a series or trilogy
Some books with similar storylines, but not the one I'm looking for:
- Deltora Quest
- The Seven Towers
- Sword of Heaven series
- Westeria series
- Diadem series