ASK-A-GEOLOGIST by Jeff Wynn

Slag — Some things you see are not what you think they are.

Not everything you see on the ground comes directly from Mother Earth — some rocks come from outer space, which I talked about earlier, and some are man-made. The following is a case in point.

Q: 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!

• (name missing)

A: Rarely is it possible to identify a rock from a photo — we usually require 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 geology 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 distinctive man-made product (the bubbles are a give-away — as long as you don’t live in volcanic provinces like Hawai’i, the Cascades, or the Aleutians). Also, your location helped me. Your rock is slag from a smelter — the by-product of melting crushed ore. After the dense metal is drawn from the bottom of the crucible, the rest is decanted and taken to a tailings pile while still liquid. To get at the copper, gold, silver, or other metals, mining engineers take advantage of gravity separation — the heavy metals end up at the molten bottom of the smelting crucible. The iridescent colors you see are fragmentary remains of those metals oxidized by the air. There was likely a mine near your home — this is common in eastern Pennsylvania.

A likely possibility is that your rock was acquired cheaply from an old mining tailings pile, ground up, and used as the bed for railroad tracks or aggregate in asphalt. Your location in Bucks County would support either conclusion. You will often see ground-up slag in railroad beds across the country and on unpaved roads, where it is generally called “road metal.”

An example of slag from a smelter (Image: bladsmithsforum.com)