It is called a Lozenge.
it merely denotes first sergeant status
In the Army and Marines, it's First Sergeant, which are both pay grade E8. In the Air Force, Master Sergeant is pay grade E7, but First Sergeant isn't a separate rank, although it is denoted with a diamond similar to what it used on Army and Marine Corps First Sergeant rank insignia. In the Air Force, the designated First Sergeant can be pay grades E7 through E9.
In the Army and Marines, it's First Sergeant, which are both pay grade E8. In the Air Force, Master Sergeant is pay grade E7, but First Sergeant isn't a separate rank, although it is denoted with a diamond similar to what it used on Army and Marine Corps First Sergeant rank insignia. In the Air Force, the designated First Sergeant can be pay grades E7 through E9.
The Army and Marines both have two ranks in the E8 pay grade - Master Sergeant, and First Sergeant. There is one E8 rank in the Air Force - Chief Master Sergeant. If they hold the position of First Sergeant, they get a diamond on their rank insignia to denote it, but their rank remains Chief Master Sergeant.
A First Sergeant or a Master Sergeant. Which rank they actually are depends on their job. The rank insignias are different. They both have 3 chevrons and 3 rockers, but the First Sergeant has a diamond in the middle and the Master Sergeant has a pair of crossed rifles on the middle. It is improper to address either one of these ranks by shortening the rank names. www.marines.com
First Sergeant (E8).
Well...every other rank that isn't First Sergeant, I'm guessing...
YES
There is no such rank as "First Class Sergeant". You have either Sergeant First Class, which is E7, or First Sergeant, which, along with Master Sergeant, is E8.
Gunnery Sergeant is pay grade E7, and the equivalent Army rank is Sergeant First Class.
Master Sergeant in the Army is pay grade E8. First Sergeant is the other Army E8 rank.