• Help ID this backyard rock


    What is this? Part of it looks like steel, my sister and me found them in our backyard underground while we were digging. -Ali, age 7

    Naturalist's Response

    Specific gravity demonstration

    Hi, Ali And Crew:

    Thanks for your post!

    Your mystery rock does LOOK like it has the rusty color of steel. That’s a good sign that it includes iron oxide, or iron that’s reacted with the air to turn brown.

    Your mystery rock also seems to have lots of tiny holes in it, and a general “popcorn” shape.

    Based on those two clues, my best guess is that you might have found a piece of slag. Slag is made of the leftovers when people melt iron ore and other rocks in furnaces to make useful metals like steel and cast iron. It involves bubbling air passing through the molten rock; when the rock cools and hardens again, the bubble tracks leave the lots of little holes and passageways.

    Another possible source for your mystery rocks could be bog iron, a naturally occurring form of iron that sometimes lends a rusty color to spring water. In the 1800’s, people would collect it for use by blacksmith shops, who’d smelt it into useful iron objects.

    Your mystery rock might have come from an old-timey blacksmith, or from an Ohio steel mill. Builders would often collect the tough slag rocks and use them to fill in holes at construction sites, railroad beds and roadways.

    If you want to investigate your rock further, try these next steps:

    • Magnet Test: See whether or not a strong magnet will stick to your rock. That’s a sure sign that it contains a lot of iron!
    • Streak Test: Find yourself an old (or unused) ceramic tile, and rub your mystery rock against the unglazed side. (That’s the side that isn’t brightly-colored, and feels rough like sandpaper.) If your rock leaves a colorful mark behind, that “streak” can be a helpful clue to identifying it.
    • Density Test: This one’s trickier! If you have access to a kitchen scale, you can measure your rock’s “specific gravity.” It’s a measure of how much matter is crammed into your rock’s size, or volume. Specific gravity can also be a helpful clue to your rock’s identity! There are plenty of great instructional videos online showing exactly how to measure a rock’s specific gravity; my personal favorite is this helpful one from Clay “Dr. Dirt” Robinson.

    Best of luck, and let us know what you find!

    -Naturalist Chris Mentrek