The instant invention is directed to the production of glass-ceramic articles from thermally crystallizable glass compositions containing, by weight, about 40-85% SiO.sub.2 and 2.5-17% N as basic constituents and utilizing such modifiers as the alkali metals, the elements of Groups IIA and IIB of the Periodic Table, boron, and aluminum to obtain practical glass-forming compositions. In general, at N contents greater than about 3.5%, the predominant crystal phase developed in situ will customarily comprise "nitrogen mullite", silicon oxynitride (Si.sub.2 ON.sub.2), and/or .beta.'--Si.sub.3 N.sub.4 solid solution. The crystal phase which has been denominated in the literature as "nitrogen-mullite" has a lath-like morphology and, hence, is useful in reinforcing the crystalline body. Where the predominant crystal phase comprises a silicate, it is believed that nitrogen is present in some manner within the silicate crystal structure. The parent thermally crystallizable glasses appear to be self-nucleating, i.e., the addition of a conventional nucleating agent such as TiO.sub.2, ZrO.sub.2, or SnO.sub.2 is not required to cause crystallization in situ. It is postulated that N may function as a nucleating agent.