The geology of Anglesey is complex and very interesting, it is often used as a destination for geologic field trips. There are four areas with outcrops of very old Precambrian rocks, in the west around Holyhead, in the centre around Aberffraw, in the east near Newborough, and finally at the coat between Menai Bridge and Beaumaris. North-west of the central Precambrian mass runs a belt of granitic rocks. Between those two belts lies a narrow tract of Ordovician slates and grits with Llandovery beds. The Ordovician rocks are heavily folded, crushed and metamorphosed. Between the eastern and central Precambrian masses lie Carboniferous sedimentary rocks, limestone, conglomerates and sandstone. More Carboniferous limestone is found on Puffin Island and on the north coast.

While there is some limestone on Anglesey, there are no caves open to the public. The crystalline rocks bear some minerals and ores, the most prominent is a patch of rhyolitic rocks which form Parys Mountain, where copper and iron ochre have been worked. The most spectacular mineral is serpentine, locally called Mona Marble, which is found near Llanfairynneubwll.