The island is 15 km long and 6 km wide on the widest spot. It is connected to the mainland by a 1,042 meter bridge, which was the longest bridge in Sweden when it was opened in 1964.
The island saw a population boom in the second half of the 19th century, due to the development of steam driven sawmills. In 1850 the island had 950 inhabitants, mainly farmers and fishermen, but by 1900 the population had grown to almost 7,000, following the establishment of 18 sawmills between 1860 and 1897.
The most famous of the Alnön minerals is the rock alnoite, named from the Island, which is a lamprophyre chiefly composed of biotite or phlogopite and melilite as essential minerals, commonly with olivine, calcite and clinopyroxene. Perovskite, apatite, nepheline and garnet may also be present. Diamonds have also been found on Alnön, but only in very small amounts.