A high-alumina glass-ceramic product with a substantial proportion microcrystalline is formed from a glass by thermal treatment, the main crystal phase being beta-spodumene or betaeucryptite, and the product having a thermal coefficient of expansion of less than 40 X 10 7/*C. At least 90 percent of the composition comprises major constituents and nucleating agents as follows, in percentages by weight: Major constituents: SiO2 55-75Al2O3 15-30 Li2O 2-15MgO 0-20 Nucleating agents: P2O5, 0.5-6.0 percent, together with one of the following metallic oxides: A. MoO3 0.5-4.0 b. WO3 0.5-4.0 c.V2O5 0.52.0 d. TiO2 0.2-less than 2. The thermal coefficient of expansion is less than 40 X 10 7. An alternative nucleating agent is cadmium sulpho-selenide.