Researchers at the University of Birmingham and at MIT have successfully used naturally forming calcite crystals to render objects invisible. In the Birmingham experiment, a paper clip was rendered invisible when a composite crystal was placed over it. By gluing together two calcite crystals with opposite crystal orientations, the latter reflected and refracted the light coming through in such a way as to conceal the paper clip on the other side, making it appear as though it wasn’t there.
Since the calcite crystals can grow well over a dozen feet long, it is theoretically possible to conceal a person or even an average New York City apartment!
source Popsci
