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Geology/help identifing these two rocks

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rocks
rocks  
I picked these two rocks up at a yard sell,don't know what they are just cought my attension.Would like to know what kind of material is in the rock and if there's any value in them.Thank you..

Answer
Hi Joseph,
You have a couple of nice looking samples there.  The one on the left is a called a "geode" and the other is the mineral "amethyst".

Let's discuss the geode first. "Geode" is Greek for "earth-like" and is a, basically, a hole filled with crystal.  What normally happens is, a process underground creates a cavity in the rock.  In igneous rock (rock that became hot enough to melt) gases can accumulate and force an open area in the melt.  Eventually the magma (melted rock underground) cools enough to solidify and form rock.  But the gas that formed the pocket created enough outward pressure so that it maintained the bubble even as the rock became solid.  The gas will leak away through very small cracks and you are left with rock, underground, with holes in it.  

At this point the rock may look solid but it has small cracks that you can't see but that water can flow through.  Groundwater can fill up the "bubbles" that the gas formed, and minerals in the water can precipitate out, as crystals, in the cavity.   Sometimes there is enough mineral in the water to entirely fill in the cavity, as in your specimen.  At other times there won't be enough to fill it and you are left with individual crystals that grow into the hole.  
This site has some good examples of geodes much like yours, but the cavities aren't filled in like your specimen.
http://www.mobileminingexperience.com/full site/Geode Cracking Programs.html


Your other specimen is "amethyst".  That too is Greek and means "not intoxicated"; it was believed that it would protect the owner from drunkenness.  As an aside, I don't think science bears that out.  Amethyst is a variety of quartz, the same material that fills your geode.  What causes the color is complicated but it has to do with natural irradiation of iron atoms in the crystal structure.  It's considered a semi-precious stone and is used in jewelry a lot.  It's also the February birthstone.  The value of the stone increases as the purple gets darker.

Besides the fact that amethyst is quartz, your two specimens are linked in another way.  The amethyst is a piece of another geode.  Amethyst is often found in geodes.  This site has some amethyst geodes, it will also give you an idea of how big some geodes are;

http://www.wegnercrystalmines.com/InventorySpecials1.html


Hope this helps.
Bob

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C. Robert Reszka, Jr.

Expertise

I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.

Experience

I have been working for the State of Michigan for 36 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.

Organizations
Michigan Basin Geological Society

Publications
Decade of North American Geology.
Bedrock Geology of Michigan

Education/Credentials
BS Wayne State University

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