Usable. I worked with someone called Gem once. Hers was actually short for
Gemini - one of the more unusual names I've come across - but she was always called Gem. I just had a look at stats here for it and it's been used more on boys than girls, but it seems fairly unisex to me:
Gem
AlexandraGem
RebeccaGem
LucyCharlotte Gem
Frances Gem
Marianne Gem