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How to Identify Rocks by Their Density

A rock's density -- its weight at a certain volume -- can help you identify a type of rock when appearance alone isn't enough to distinguish it from other varieties. Each rock has its own density, ranging from the low density of coal all the way up to hematite, a very dense iron oxide mineral. If you're stumped on a rock's identity, you can weigh and measure the rock and then match it up to known rock densities.

Things You'll Need

Small bowl
Large bowl or pail
Measuring cup
Small scale

Instructions

- 1
  Measure the volume of the rock. Fill a small bowl to the brim with water. Place the bowl in an empty pail and then dunk the rock in the water. Carefully remove the small bowl with the rock and what water is left in it. Pour the water from the pail or large bowl into a measuring cup to determine how many milliliters there are. Write this measurement down. In this example, the rock displaces 4 ml of water.
- 2
  Place the rock on the scale and note its weight in grams. Write this figure down. In the example, the rock weighs 10 grams.
- 3
  Divide the rock's weight in grams by its volume in milliliters (a milliliter is equivalent to a cubic centimeter). In the example, the rock comes out to a density of 2.5 (10 divided by 4).
- 4
  Compare the density to the chart on the Austin Powder or EduMine website, which lists the densities of common types of rocks, to see if your rock matches up to any of them. In the example, the density narrows down the identity of the rock to a handful of types, including asphalt, pyroxenite and slate.
- 5
  Use a rock identification key, such as the one at the rock-collecting website Rockhounds (see Resources), to visually identify the rock and separate it from the other rocks of similar densities.

Rocks & Minerals