My message was bounced back by the server. Not sure it came through. Just in case, I repeat it here. Apologies for multiple posting. Hi, There is a definition of crystal given by the International Union of Crystallography (IUCr): "A material is a crystal if it has essentially a sharp diffraction pattern." http://reference.iucr.org/dictionary/Crystal see also: Acta Cryst. (1992), A48, 928 "In the following by 'crystal' we mean any solid having an essentially discrete diffraction diagram, and by 'aperiodic crystal' we mean any crystal in which three- dimensional lattice periodicity can be considered to be absent. As an extension, the latter term will also include those crystals in which three-dimensional periodicity is too weak to describe significant correlations in the atomic configuration, but which can be properly described by crystallographic methods developed for actual aperiodic crystals." There was a long discussion whether this definition is complete enough, but the 'official' definition is still there. See https://www.degruyter.com/view/j/zkri.2007.222.issue-6/zkri.2007.222.issue-6/zkri.2007.222.issue-6.xml for the discussion on the definition of crystal by leading world crystallographers. The papers on the topic are free to download. With best wishes, Sergey