Phenocrysts

Phenocrysts are large, conspicuous crystals found within some igneous rocks, like granite or volcanic rocks. They form early in the cooling process when mineral crystals begin to develop in magma or lava. Because they crystallize more slowly than the surrounding material, they grow larger and stand out against the finer-grained matrix. Phenocrysts provide clues about the history of the rock's formation, including the rate of cooling and the composition of the magma. Their presence and size can vary depending on the cooling environment and the mineral types involved.