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Slag

Gallery of Artificial Rocks

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The mineral waste product of smelting and roasting operations is called slag. Widespread near cities, it is often mistaken for lava or meteorites. (more below)
Mistaken for lava or meteorites
Photo courtesy Chris Soeller, all rights reserved
Click the photo for a larger version. Slag has a variety of sources and a variety of appearances; see the next two pictures in this series for more examples. Several things distinguish slag from real rocks. Unlike lava, slag never forms flows or outcrops but is always found in piles or scattered in the dirt. It occurs in places with a former history of iron smelting or metal mining, even if that was centuries ago. It may be associated with industrial zones, or it may be in an old landfill. Slag never has crystals in it, whereas many rocks do. Slag is softer and more crumbly under the hammer than real lava. It usually tarnishes readily in mottled patterns and may present garish or strange colors. It gives the overall impression of being a waste product, without a real rock's integrity, for lack of a better word. This specimen has bubbles, but unlike the vesicles of real lava, they are irregular and without even a fractal order.

Slag may be a byproduct, but it is not useless waste. The National Slag Association wants you to make friends with it.

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