Does it mean anything if a girl calls you a gem?

[FeEl GoOd]

So I hang out with this girl almost everyday....she is my best friend, and personally I love her, we have a weird relationship we flirt a lot and touch each other.....a lot recently.........she said she was talking to her mom about how great i am and p.s. I don't really think im that great, and she said her mom called me a gem.....and she said she agreed........what does being called a gem really mean? im 19 shes 20 if it matters