When someone is really precious and adorable ie nerdy and cute. They just make you want to say aaaw because they are such a cute little gentleman. They are
not fit and you don't fancy them (
diamond) but they are almost as precious hence
gem.
Holly- Harry just held the door open for me
Eleanor - what a gem
OR
Libby -
Connor fell over in the hallway today
Jess - what a gem