Lake Superior agates

These may be the oldest agates on Earth, and they exist because North America once tried to tear itself in half.

Lake Superior agate is an iron-coloured agate from the Lake Superior region of the United States and Canada. It shows up as a collectible beach stone along the shoreline in Minnesota, Michigan, Wisconsin and Ontario, and it also turns up inland in glacial deposits across those states and several others in the Midwest, carried there by ice. Collectors prize it for vivid banding in reds, oranges and whites, though other colours occur.

The rift is the whole story. Roughly 1.1 billion years ago, in the Mesoproterozoic, the North American craton began to split along a rift some 2,000 kilometres long, running from what is now Lake Superior southwest as far as Kansas and southeast into Lower Michigan. The split never finished, but it poured out enormous volumes of iron-rich lava, and the agates formed as nodules inside those basaltic flows. The iron in the lava is why the bands are red.

Geologists have been working on that rift for over a century, and the Midcontinent Rift is now recognised as one of the best-preserved large igneous provinces of Precambrian age anywhere. As James D. Miller Jr of the Precambrian Research Center at the University of Minnesota Duluth sets out, that recognition rests on three kinds of hard data gathered over recent decades: high-resolution aeromagnetic surveys, deep-crustal seismic reflection profiles, and petrochemical and radioisotopic work. Between them they have transformed understanding of the rift's three-dimensional structure and how it evolved.

Minnesota made it the official state gemstone in 1969.

Beach hunting is the accessible way in. Wet stones show their banding far better than dry ones, so walk the waterline, and check local rules before filling a bucket, because collecting limits and outright bans vary between shorelines, parks and reserves around the lake.