Covalent crystals are an extreme example of a large molecule--they are highly-ordered 3-dimensional arrangements of trillions of trillions of atoms connected by covalent bonds. These materials have high melting points and good mechanical strength. The most familiar examples are diamond and graphite, both composed of purely carbon atoms (just connected in slightly different geometries). Other examples include silicon, quartz (silicon dioxide), and silicon carbide; however, because silicon is actually a metalloidrather than a nonmetal, these latter materials are entering the "grey area" between the different material and bond types.
