Jem. But be prepared for "... and the Holograms" jokes from anyone who grew up in the '80s.
Famous people named Jemma:
Actresses Jemma McKenzie-Brown and Jemma Redgrave; model Jemma Wellesley; pro wrestler Jemma Palmer; Olympic swimmer Jemma Lowe.
For now, Jemma is most popular in Australia.