Well, first off, the kind of magic that Ruby start getting is just basic teleportation (maybe with other people along), slight regeneration, and levitation. Since she's only worn it for a short time, it wouldn't take effect until after a certain amount of time, these magic abilities would start to be evident through her accidental use of it. Also, it's influence is like a drug, Ruby would start to feel protective to keep the pendant on, causing her to change drastically in emotion when someone would try to take the pendant away from her.

This is pretty much what I assumed it's uses and effects were, since it's never really told much.