Akin to European soft-paste porcelain, this material, known as fritware or stone-paste, is described in the fourtheenth-century treatise of Ahu'l Qasim as consisting of ten parts ground quartz, one part ground glass and one part fine white clay.
Source: wiktionary
Mason and Tite propse that Islamic stone-paste material may have been developed by migrant Iraqi potters in Egypt in the +10ᵗʰ to +11ᵗʰ centuries, after some initial experiments with adding powdered glass to clay bodies in Iraq in the +9th century.
Source: wiktionary
Later tiles were made from a compound called “stone-paste”—a mixture of ground quartz, glaze frit, and white clay.
Source: wiktionary