The matrix or groundmass of rock is the fine-grained mass of material in which larger grains or crystals are embedded.
The matrix of an igneous rock consists of fine-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."
The matrix of sedimentary rocks is a fine-grained clay or silt in which larger grains are embedded.