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".
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica (11th ed.). Cambridge University Press. p. 332.