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How to Measure Rock Density

Density is a basic scientific concept. Defined, mass divided by volume equals density. Mass is the weight of an object and volume is the size of an object. The resultant refers to the object's weight per unit of volume, or its density. It is expressed as grams per cubic centimeter (g/cc or g/cm³). You can identify the name and composition of a rock if you know its density. Rocks have a density of around 3 g/cm³. For example, basalt has a density of 3 g/cm³, granite has a density of 2.7 g/cm³ and sandstone has a density of 2.3 g/cm³.

Things You'll Need

Granite rock
Spring scale
Container of water
Calculator

Instructions

- 1
  Suspend a piece of rock from a spring scale. Take a reading from the scale. The resultant number is the weight of the rock in grams. This is the object's mass. For example, the rock weighs 670 grams.
- 2
  Lower the rock into a full container of water to calculate its volume. Take a reading from the spring scale. The resultant number is now 420 grams.
- 3
  Subtract 420 grams from 670 grams. This gives you a reading of 250 grams, which is the amount of water that was displaced by the rock. One gram of water has a volume of 1 cm³. The volume of displaced water approximates the volume of the rock, which is 250 cm³.
- 4
  Divide the mass by the volume to find the density of the rock. For example, 670 grams divided by 250 cm³ equals 2.68 g/cm³, which is the density of granite.

Rocks & Minerals