Master's Regalia
The Master's Apron is adorned with the jewel of his office, the Sqaure.
The Apron represents that the wearer is elected into the office of Master, however the wearer of the Jewel is the one currently taking on the role of Master.
The only person in a lodge that wears a hat -- often times a tophat or, modernly, a fedora -- is the Master, this is again to signify his station as the leader of the lodge. But also represents that once he takes off the hat and passes it on to someone else he is still equal with the other brethren.
The Gavels, which are exclusive to the Principle officers, give the Master the authority to call the craft up to attention.