Abstract: A cordierite-type glass-ceramic and a process for its preparation are disclosed, characterized in that a nucleating agent in a small amount is added preferably to a glass batch capable of producing cordierite. The nucleating agent aids in forming a glass-ceramic refractory of relatively large amounts of crystalline cordierite. The melt is quenched and ground to an average particle size no greater than about minus 325 U.S. Standard sieve. This particle size in combination with a relatively small amount of a nucleating agent has been found to increase the amount of cordierite formed during crystallization and simultaneously decrease the amount of glassy phase remaining in the glass-ceramic. In this manner, the ultimate product more nearly resembles pure cordierite. The nucleating agent includes the compounds, usually the oxides, of Mo, Ta, Zr, Nb, Ti, Li, As, and mixtures thereof. The preferred nucleating additive or agent is Nb.sub.2 O.sub.5.