RockHoundR

Slag is the glassy by-product of smelting ore to recover metal. It is typically vitreous and full of gas bubbles, often with a very rough exterior, and it may attract a magnet because of metal left behind. Many rocks photographed and sent to a meteorite researcher as possible meteorites turned out to be slag, and slag has no fusion crust. Its main components are oxides of calcium, magnesium, silicon, iron and aluminium, and it is reused in road construction and landscaping gravel.

Luster
Vitreous

Is this slag?

3-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

  • 1
    Read the texture
    Look at a fresh break with a hand lens. Texture: Glassy, with vesicles (gas bubbles); exteriors often very rough. Grain size: glassy.
  • 2
    Pick out the minerals
    Composition: Oxides of calcium, magnesium, silicon, iron and aluminium; may contain metal grains. Identify the two or three most abundant grains.
  • 3
    Read the luster
    Hold the specimen under a strong light. Luster: vitreous.

Often confused with

Slag vs. its common look-alikes, and how to tell them apart in the field.

Two stony meteorite fragments showing black fusion crust over a pale interior
igneous
Meteorite
Mohs 5-8

How to tell apart: Luster reads vitreous on Slag and metallic to dull on Meteorite.

Compare →

All properties

Density
2.4 g/cm³
Luster
Vitreous
Uses
Construction
Host rock
Not a natural rock: a by-product of smelting ore
Texture
Glassy, with vesicles (gas bubbles); exteriors often very rough
Grain size
glassy
Composition
Oxides of calcium, magnesium, silicon, iron and aluminium; may contain metal grains

Where rockhounds find slag

11 mapped spots

Classic worldwide localities

  • Road construction fill and landscaping gravel

Field-hunting tip

Where it forms: Not a natural rock: a by-product of smelting ore. In the U.S., the densest reported localities are in Michigan, Ohio. Start trip planning there.

Browse 11 U.S. spots for slag →

More slag photos

  • Vesicular grey slag showing the gas bubbles that distinguish it from meteorite
  • Vesicular slag lump found in the field

Common questions

How do you identify slag?+
Texture: Glassy, with vesicles (gas bubbles); exteriors often very rough. Composition: Oxides of calcium, magnesium, silicon, iron and aluminium; may contain metal grains. Luster: vitreous.
Where is slag found?+
Notable localities include Road construction fill and landscaping gravel.
Can I find slag in the United States?+
RockHoundR maps 11 slag rockhounding spots across 2 U.S. states. The top states are Michigan, Ohio.
Is slag safe to handle?+
Slag contains toxic metals. Slag from electric arc furnaces can contain toxic metals.
What rocks look like slag?+
Slag is most often confused with Meteorite. A quick hardness test and a streak check separate the look-alikes faster than color alone.
What kind of rock does slag form in?+
Where it forms: Not a natural rock: a by-product of smelting ore. Working float back to the host body is the standard way to chase a fresh occurrence.
What is slag used for?+
Slag is used in construction.

Find slag on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.

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