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Question
Rock
Rock  
I found this dark green rock in our garden.  Could you shed some light in what it is?

Answer
Hi Kim,
Very interesting specimen. You have an example of glass slag. It's basically rough glass from an iron blast furnace or glass making operation.  Much of it originated in England in the late 19th century and It is thought that manufacturers created slag glass by taking slag, the waste content of metal ores, from iron-smelting works and adding it to molten glass. The process created a variety of effects, such as tortoiseshell and marbling.

This may be a sample from a more current glass making operation, although slag from an iron furnace can be very glassy.

It can have value as an antique since much was produced in the 19th century from iron smelting factories and from glass making facilities. It was used for commercial purposes and for lamps, glassware items, and jewelry.  Slag glass is often found in purple and sometimes in blue, brown and green, with streaks and swirls of white or cream. The glass originally went by descriptive names, such as “Marble Glass” or “Brown Malachite.”

I don't know how old your sample is or if there were any facilities like I described near the area. You might be able to find out more if you take it to a local university or rock club.

Here's a picture that looks much like yours but without much of the surface discoloration.

http://www.forginglinks.net/2011/05/glass-and-iron/

Hope this helps.
Bob

Geology

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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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