Unlike ordinary crystals, the materials that Shechtman was studying lacked periodicity, meaning their atomic structure could not be depicted by a geometric pattern of atoms that repeats in three dimensions at fixed intervals. Nonetheless, they were ordered crystals—sort of.

Shechtman showed that these specimens were oddly ordered materials, which later came to be known as quasicrystals. The class of materials includes a large number of multicomponent alloys that often exhibit five- or 10-fold rotational symmetry, a condition that’s forbidden in conventional crystallography. The very definition of crystallinity included periodicity of the crystal structure.