The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.

The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals (phenocrysts) are embedded. This porphyritic texture is indicative of multi-stage cooling of magma. For example, porphyritic andesite will have large phenocrysts of plagioclase in a fine-grained matrix. Also in South Africa, diamonds are often mined from a matrix of weathered clay-like rock (kimberlite) called "yellow ground".