Sunday, July 12, 2009

Great Rock!

Yesterday, Larry Feist from Grand Rapids came into the museum with an unusual 5.5 pound rock that he found on the local beach in Grand Marais. When it was dry, it didn't look like much of anything. But when we wet the rock, color and some banding popped out everywhere.

My guess is that it is a brecciated chalcedony agate, with pockets and banding of amethyst (purple) and chrysoprase (green). Brecciated rocks form when little pices of angular rock are cemented back together in a matrix. Chrysoprase is similar to carnelian (red translucent chalcedony), but instead of the microcrystalline quartz being "stained" with iron oxide, it is stained with nickle oxide. Since there is a lot of nickle up in the Sudbury, Ontario area, it is likely that this rock originated from there, and was dragged by the glaciers and/or icebergs to the Grand Marais beach.

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