Although the first thing you do in geology (among many "first things") is to learn to see rocks, one of the first things you need to do in looking at rocks is to beware of various manmade objects that are commonly mistaken for rocks.
For instance, look at slag. Slag is not a rock, just the solid waste product from smelters of various kinds. But at least once a week, I hear from readers with pictures of slag wondering if they have found a meteorite, or lava. I'm an experienced seer of rocks, and to me slag looks like imitation lava just as obviously as those fake stones made for hiding your house keys look like imitation rocks. Get to know more of these not-rocks in the artificial rocks gallery.
But I will admit to doing double-takes sometimes, even now, at pieces of concrete. Concrete is imitation conglomerate.
Slag, from the artificial rocks gallery
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