Ask NRRI

"How can I identify an agate?"

NRRI Geologist Steve Monson-Geerts answers:

First of all, in order to be able to identify a true "Lake Superior Agate," one must know a little about the origin of these beautiful semi-precious gemstones. Besides being Minnesota’s official gemstone, Lake Superior agates originate from the rocks that surround the present day Lake Superior basin. These distinct agates were formed about a billion years ago when basalt magma from the earth’s mantel, flooded out on to the surface as lava flows, as part of the mid-continental rift. Spherical cavities were formed in the lava flows when gases and other volatiles were trapped during cooling, and provided the empty spaces necessary for silica-rich fluids to collect and deposit layers of quartz. These layers of quartz with varying amounts of impurities (mostly iron) formed the unique banding pattern that Lake Superior agates are known for.

Things to look for in the identification of Lake Superior agates, include; an overall irregular and spherical shape (from being formed inside a gas bubble), bubbly pitted exterior texture and a glassy or waxy appearance. The agates can range in size from as small as a pea to many pounds, with an average about the size of a golf ball. Depending on whether the agate has been broken, you may see the thin concentric alternating bands inside, ranging from clear or white quartz to deep red jasper. Some agates are semi-translucent if held up to sun light. Other colors that may be present in Lake Superior agates include various shades of yellow, orange and even violet.

The location from which an agate is found may also give us a clue to whether or not it is a Lake Superior agate. Since agates are liberated from the basalt in which they formed by means of wave erosion and glaciers, the chances are good that it is a Lake Superior agate if it has been found on the beaches of, or within gravel pits in and around and south of Lake Superior. Some of the largest and most beautiful specimens have been found near Moose Lake, Minnesota about 45 minutes south of Lake Superior.