1. An occurance of high hilarity. This may be a phrase, action, put down or accident.
2. A precious stone carved into an amusing shape, like a phallus.
Richie: "Chris you douchebag - I'd try and make fun of you only I don't want to compete with nature!"
Tom: "Ha! That's a comic gem!"
"Look at this funny looking diamond in the shape of a plump rump - it's a comic gem!"