File:Agate nodule ("Lake Superior Agate") (Jo Daviess County, Illinois, USA) 16 (34783567065).jpg

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Agate ("Lake Superior Agate") found in Illinois, USA.

"Agate" is a rockhound/collector term for cavities in rocks (usually sedimentary rocks such as limestone or igneous rocks such as basalt) that have been partially or completely filled with irregularly concentric layers of microcrystalline, fibrous quartz (chalcedony - SiO2). Agate is quartz.

The above specimen is "Lake Superior Agate", which is the state gemstone of Minnesota. It is a variety that occurs around Lake Superior in Minnesota and the Upper Peninsula of Michigan. The agate filled large vesicles (gas bubbles) in ~1.1 billion year old basalt lava flows of Minnesota's North Shore Volcanic Series and Michigan's Portage Lake Volcanic Series. The reddish-brown and yellowish-brown coloration seen in this agate sample is the result of iron oxide impurities in the chalcedony layers.

This sample was actually near Dubuque, Iowa - way south of Minnesota. Pleistocene glaciers and post-glaciation meltwater rivers transported some agates from the Lake Superior area to deposits along the Mississippi River.

Locality: undisclosed site attributed to the Mississippi River in western Jo Daviess County, northwestern Illinois, USA
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Source Agate nodule ("Lake Superior Agate") (Jo Daviess County, Illinois, USA) 16
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/34783567065 (archive). It was reviewed on 10 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

10 October 2019

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Date/TimeThumbnailDimensionsUserComment
current07:14, 10 October 2019Thumbnail for version as of 07:14, 10 October 20191,691 × 1,601 (1.6 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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