Long ago (15 years?), I partially read a novel which I really liked. I don't remember the title or full plot of it. I would be grateful if some one can identify it from the limited plot below:
Three (or four) teenagers are taken from their worlds (or different places on earth) to a strange world full of trolls, gems, powers etc. Each one gets attracted a particular colored gem which gives them a particular power. They keep going forward battling all obstacles to achieve some vague objective.