A piece of jewelry worn on the ring finger made of low-grade gold and Cubic Zirconium that looks similar to an engagement/wedding ring. Worn by unmarried women (usually very attractive or very smart) on college campuses informing many would-be suitors they are not seeking a potential SO.

These can be impossible to distinguish at a distance with the naked eye from real wedding or engagement rings as you have to talk to and get to know the female in a non-threatening way (very difficult as they can see a pending advance coming from a mile away).

This mind game tends to have a very high success rate on campuses (this is where my sister learned about the concept).