Well, I guess everyone has distinct points of view, but the team finding it really weird to have a gem of doom and thus wondering if it's real or not sounded really logical to me. I'd say that the opposite (assuming without an hesitation that they now own such a rare thing) would be way more of an immersion breaker.
Yes, I would agree with you if it was some random mission that they beat they. clearly stated this mission. was a mission that thousands. of other characters could not figure out. this mission was difficult. If they just found it under normal circumstances.
what would be more rare. to get get 1 of the 10 gems or to be the first to get a fake gem. if they where fake gems you would get it from a trader not the reward box it would be online post saying some others getting fake gems from a reward box and they would know its a chance