To most people, crystals mean diamond bling, semiprecious gems or perhaps the jagged amethyst or quartz crystals beloved by collectors. To Norman Yao, these inert crystals are the tip of the iceberg. If crystals have an atomic structure that repeats in space, like the carbon lattice of a diamond, why can’t crystals also have a structure that repeats in time? That is, a time crystal?
https://www.eurekalert.org/pub_releases/2017-01/uoc–sun012617.php
