I've recently learned that there is some kind of tradition to trade coins in the military that seems to have begun in distant past, some say WWI. It would seem that if someone is impressed with you, either as an individual or as a unit, that they would give you a coin. The coin has some sort of logo on it, either a unit logo, an exercise or operation, or can even be a coin from an individual, such as a general officer.
It would further seem to be the case that when you enter a bar you are required to produce a coin and whoever has the least impressive coin has to buy the drinks. I suppose this is some kind of ground pounder thing, we never did this in aviation. However, I do have two coins that aren't of any particular merit.
The first coin can reasonably be called a coin from its size. It's 1.5" in diameter. Like most of these trinkets, it is crudely minted and is much heavier and thicker than anything I'd like to carry around. This was given to all the people in my squadron in commemoration of our renaming after taking on the F/A-18D Hornet. Prior to this ceremony, we were VMA(AW)-242, The Bats. When we got rid of our A-6E Intruders, we became VMFA(AW)-242. We were still the Bats, but our commanding officer changed our logo to include a faux Latin motto, "Mors ex Tenebris" or "Death from the Darkness." Here's the coin they gave me.
-242Obverse.jpg)
While I was overseas, we celebrated the 213th Anniversary of the Marine Corps. For the second year in a row, I was the youngest Marine present at the officers ball (the previous year I was the youngest present at the entire 3rd MAW ball!). That meant that I had to go up on stage and get the first slice of cake. Anyway, they gave out "coins" to all of us. This one is much too large to be carried around, it's about 3-7/16" in diameter and quite heavy.

Here's a comparison of their sizes.
-242.jpg)
So, I guess that I and everyone else in my squadron, not to mention my Groups and Wings completely lost the supposed importance of these trinkets. I have no plans to carry this around, they will stay on my coffee table. For more examples go to this web site.