Lake Superior Agate

January 15, 2016


Photographer: Dale Hugo
Summary Author: Dale Hugo

Many different types of agates exist on Earth. The ones shown above, from my personal collection, are peculiar to the Lake Superior region. They can be found distributed in gravel deposits throughout the upper Midwest as a result of ice age glacial erosion processes.

The agates I collected, the biggest is about 3 in (7.5 cm) in diameter, show the color typical of the Lake Superior variety. The reddish tone is due to the presence of iron. But what causes the bands? There are two main theories regarding the banding. One is that the gas pockets left in cooling magma were infiltrated by successive shells of chalcedony that hardened as the pocket cooled forming the layers. The second theory is that they were created by differential separation in a gel called a Liesegang ring. In this way, iron and other salts are diffused and then deposited in rhythmic bands.

Agates are considered semiprecious stones and are prized for use in jewelry by hobbyists. Additionally, Lake Superior Agate is Minnesota's state mineral. Note that the four agates in front have been polished by a rock tumbler.

Photo Details: Camera Maker: Apple; Camera Model: iPad 2; Lens: Apple; Focal Length: 2.0mm (35mm equivalent: 44mm); Aperture: ƒ/2.4; Exposure Time: 0.033 s (1/30); ISO equiv: 80.