Density & specific gravity


Density is the term for how heavy an object is for its size. Rock density is usually expressed in units like grams per cubic centimeter (g/cc or g/cm3), kilograms per cubic meter, or pounds per cubic inch (cubic foot or cubic yard).

Rocks vary considerably in density, so the density of a rock is often a good identification tool and useful for distinguishing terrestrial (earth) rocks from meteorites. Iron meteorites are very dense, 7-8 g/cm3. Most meteorites are ordinary chondrites, and ordinary chondrites have a density half as much as irons. Most ordinary chondrites are in the range 3.0 to 3.7 g/cm3, which is denser than most terrestrial rocks. For example, limestone (2.6 g/cmor less), quartzite (2.7 g/cm3), and granite (2.7-2.8 g/cm3) are all common low-density rocks. Some meteorites have low densities (<3.0 g/cm3), but such meteorites are rare. The density of basalt, one of the most common types of terrestrial volcanic rocks, can be as high as 3.0 g/cm3. The only types of terrestrial rocks that are denser than meteorites are ores – oxides and sulfides of metals like iron, zinc, and lead. For example, rocks composed of hematite or magnetite (iron oxides) are often mistaken for meteorites (see concretions). Such rocks have high densities, 4.5-5 g/cm3, which is greater than that of any type of stony meteorite.

Specific gravity

In order to measure density, it is necessary to measure the volume of a rock. That is hard to do accurately. Just as useful as density, however, is the specific gravity. Specific gravity is the ratio of the mass (weight) of a rock to the mass of the same volume of water. Water has a density of 1.0 g/cm3, so the numeric value of specific gravity for a rock is the same as that for density. Because specific gravity is a ratio, it has no unit.

Specific gravity is easier to measure than density. In order to measure specific gravity you need a balance or scale with a hook on the bottom. The technique is described in most high school physics books and most high schools (general science and physics labs) would have a balance that could be used for measuring specific gravity. It may be difficult to obtain an accurate measure for a small rock, e.g., <10 grams.

Bottom line

If you have a rock that is not metallic and it has a specific gravity greater than 4.0, then it is not a meteorite.

If you have a rock that has a specific gravity in the range 3.0 to 4.0, it might be a meteorite. That is the good news. The bad news is that if you collect 1000 rocks with specific gravities in that range, they are probably all earth rocks because many types of earth rocks are in the 3-4 range. If you have a rock that has a specific gravity of less than 3.0, it is almost certainly not a meteorite. Many types of earth rocks have specific gravities less than 3.0.