I don’t have any specifications about the meanings of gems. But I only know is that it is only for the purpose of beautification and jazzing it up. You can buy these wonderful looking gem stones or rhinestones on local market or shop that sells also ribbons and other sewing material stuffs.
It is best to use quick dry glue or mighty bond when attaching a gemstone or rhinestone into your tambourine. We often do have some bloopers when we dance and our gems detach themselves on the tambourine. Well, personally you can do without it. But as what I have said, it is for the sole purpose of beautifying your tambourine. Picture below is for representation only.