MINERAL OF THE MONTH
April 2008: Shadow Agate
The mineral of the month is my favorite agate: the shadow agate. This specimen was picked up off the Grand Marais beach by the museum founder, Axel Niemi. He found it in 1925 when he was just eight years old. He had it for almost 40 years when he finally sliced the agate, discovering the intricate banding. The best shadow agates in the world come from Lake Superior, Botswana Africa, and Queensland Australia.
Shadow agates exhibit an optical effect of movement across the bands. Depth is perceived from light penetrating and bouncing between alternating clear and opaque layers. When you move these agates back and forth, shadows can be seen racing across the surface. Many factors contribute to the shadow phenomenon including the regularity, contrast, distance, and depth of the bands. When the right conditions exist, light disappears into the clear chalcedony bands and is not reflected back out to the eye. When little or no light is returned to the line of sight, we interpret this as a dark region, or shadow.
Mineral of the Month Archives
May 2007 Rainbow Fluorite
June 2007 Lake Superior Michipicoten Agate
July 2007 Labadorite
August 2007 Rain Flower Agate
Fall 2007 Malachite
December 2007 Nepheline Syenite
January 2008 Native Copper
February 2008 Amazonite
March 2008 Lake Superior Agate
April 2008 Shadow Agate
May 2008 Apohpylite
June 2008 Ocean Jasper
Summer 2008 Marra Mamba Tiger's Eye
September 2008 Mohawkite
October 2008 Mexican opal
November 2008 Prehnite
December 2008 Picture Jasper
January 2009 Sea Shell Jasper
February 2009 Polychrome Jasper
March 2009 Selenite Desert Rose
Spring 2009 Coyamito Agate
July 2009 Obsidian Needles
August 2009 Goethite
September 2009 Banded Iron Formation
Fall 2009 Fairburn Agate
February 2010 Kentucky Agate
March 2010 Fossilized Dinosaur Bone
April/May 2010 Kentucky Agate
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Gitche Gumee Museum.
E21739 Brazel Street
Grand Marais, Michigan 49839
