Saturday, January 7, 2012


Not everything you see on the ground comes from Mother Nature - some rocks come from outer space, which we'll talk about later, and some are man-made. The following is a case in point.


My mother and I recently found this rock buried in our garden in
Bucks County, PA. I've been searching the Internet, trying to
identify it, but nothing seems to match. It's fairly small, about
1.5"x2". It is very smooth, a bluish-green with darker and
lighter layers, and a few large air bubbles. I've never seen a
rock like this, and it seems a bit out of place. Please help!
Thank you, very much!


Rarely is it possible to identify a rock from a photo - it usually requires examination with a hand-lens, an acid bottle, a streak test, and a knife scratch to verify hardness of individual grains. This doesn't work in all cases, by any means. An old Russian (geologist) proverb goes something like this: Один обнажение, два геолога, три мнения (translation: one outcrop, two geologists, three opinions.) In these cases it may require examination of a thin-section of the rock under a light-polarizing petrographic microscope. Even then, a chemical analysis or even a scanning electron microscope may be needed to be absolutely certain what you are dealing with.

However, in THIS case your photo shows a rare, distinctive man-made product (the bubbles are a give-away as long as you don't live in Hawai'i or the Aleutians). Your rock is slag from a smelter - the by-product of melting crushed ore, decanting it while still liquid to get at the copper, gold, silver, or other metals which follow gravity and end up at the bottom of the crucible. There may have been a mine nearby your home. Another common possibility is that it has been acquired cheaply from an old mine, ground up, and used as aggregate in asphalt by a road crew. Your location would support either conclusion. You will also see ground up slag in a lot of railroad beds, where it is sometimes called "road metal."


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