Matrix (geology)

The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.
The matrix of an igneous rock is made up of fine-grained, often tiny, crystals in which larger crystals, called phenocrysts, are embedded. 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 made up of finer-grained sedimentary material, such as clay or silt, in which larger grains or clasts are embedded.