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Geology/is this an obsidian,tektite or impact glass?



I found this "mineral" about 22 years ago in an open clay field near my parents home in Puerto Rico. I really hope its meteor related, sounds better than lava rock in P.R..
Description:  black glass like, one section is crusted and has bubbles or gas pockets all around. I do have more pictures showing all details but i was only able to send 2. I really do appreciate your help. Thanks. im going to send a second email with 2 more pictures.

Hi Carlos,

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.

Hope this helps.


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C. Robert Reszka, Jr.


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.


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.

Michigan Basin Geological Society

Decade of North American Geology.
Bedrock Geology of Michigan

BS Wayne State University

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