Wvmarle, thanks for your response. I like the idea of some form of pinging between the two devices. I’m thinking about trying out both active and passive RFID. Even though they only offer a binary value like, “chip read” and no distance values, I was thinking some more about my project and can probably achieve what I want with some properly timed animations instead of relying on a distance value.
To give this topic context, my project is nothing special, I’m just planning on building a prop for my next Halloween costume. I know I’m way ahead of schedule on this one 
I was recently watching Indiana Jones and the Temple of Doom and now I want to recreate the three glowing stones that he was after in the movie.
The three stones (or just two) glowed when they were brought together. I was going to create a stone out of clay, then make a silicon mold of that. I’ll 3D print a casing to hold the electronics which will be inserted into the mold and then I fill the outside with a colored resin or polymer of some sort.
For the electronics package, I’ll use some form of little Arduino, with a battery pack connected to an LED array of some sort and using RFID to constantly check if the other stones are near by. If so, I’ll animate the LEDs slowely turning on and reverse the animation when the RFID can no longer sense another stone nearby.
That’s really the high level concept right now. It’s just floating in my head but I’d love to experiment with the electronics soon and see where I can take this.