Why would someone put their finger on your engagement ring? I find that really unclassy and just plain disgusting?

Everyone knows that a finger is something that naturally creates oils so why would you do that?
So far two of my collegues at work rubbed their finger on my stone while looking at it at work?
It annoyed the hell out of me.
However it happened so fast I didn’t have time to respond to it.
Am i just freaking out unnecessarily ?
By the way it is a diamond engagement ring