Searching for uncopyable key made of sparkles in plastic

Several months ago, I read about someone who was making a key that was difficult if not "impossible" to copy. They mixed sparkly things into a plastic resin and let them set. A camera would take a picture of the object and pass the location of the sparkly parts through a hash function to produce the numerical key represented by this hunk of plastic. That numerical value would unlock documents. This was thought to be very difficult to copy because the sparkly items were arranged at random. Arranging all of the sparkly parts in the right sequence and position was thought to be beyond the limits of precision for humans. Can anyone give me a reference to this paper/project? Thanks! -Peter