Polarised light micrograph of a thin section of basalt, an igneous rock. In this image, large coloured crystals appear embedded in the fine-grained basalt matrix. Such crystals are known as phenocrysts. The size of a phenocryst depends on the rate at which it was cooled out of the magma; the larger the crystals the slower the process. Magnification x15 at 35mm size. .