Experimental Soft Condensed Matter Group Harvard University, Prof. D. A. Weitz Cool Picture of the Moment

These are 2.3 micron diameter colloidal particles, dyed with a fluorescent dye, and viewed with a confocal microscope. They spontaneously form crystals, generally hexagonally closed packed (HCP) or random hexagonally close packed (RHCP). This picture shows a cross section through several different crystal domains. In the usual cross section, an HCP crystal is a simple hexagonal array of particles. In a different slice, an ordered array of squares shows up. It's possible that the growth of these domains shown above was influenced by the coverslip of the microscope slide; it's not clear if square domains can form within the bulk of a crystal, and the cross section through a RHCP structure would not show squares. Picture provided by Eric Weeks.

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Also, in some cases these particles form amorphous structures, analogous to a glass or a supercooled fluid:

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