Gloria only realizes the diamonds in the tiara are real because Mitchell gets a clean cut from them, despite having been very up close with the tiara. In "Sleeper" (S5E21) Gloria was able to recognize a real diamond in Phil's hand from several yards away.
Gloria stops the car and turns off the motor short of the premise's gate in order to sneak back in as quiet as possible, but then Gloria and Mitchell both slam shut the car doors creating excessive noise.