These major intrusive or ‘plutonic’ rocks were formed between 580 and 480 million years ago by molten rock cooling and solidifying between the Earth’s surface. The granites are only visible once the overlying rocks have been eroded away. The dark rocks, known as gabbros, are the oldest, and are rich in iron and magnesium. The true granites, visible along the northwest and southwest coasts of Jersey, are lighter in colour, and consist of three main minerals: quartz, feldspar and mica. On the southeast coast, where granites have broken through the Earth’s surface into older gabbros, a mixed rock called diorite has formed. The size of the crystals in the granite depends on the rate at which the granite cools, coarse granite has cooled slowly forming large crystals.