Crystals is played on a square grid - 21x21 is a suggested size. Players take turns marking squares with their symbol (e.g. X and O). These are called "atoms." When a player can form a "crystal" they outline it, shade it, and score as many points as there are squares in it. A crystal is any shape that has at least four atoms of one player in it, none of another player, has reflective symmetry about both orthogonal and both diagonal axes, and obeys a few other rules. The winner is the player with the most points once it is no longer possible to form new crystals.