What Is Specific Gravity

The specific gravity of a substance is a comparison of its density to that of water. Imagine a gallon bottle filled with water, a second filled with feathers, a third filled with lead weights. There are equal volumes of material present, but the bottle with the feathers will weigh less than that containing water; the bottle with lead weights will weigh the most. In order of increasing gravity, these materials would be: feathers, water, and lead. Specific gravity can be measured precisely, or estimated by a comparison, as above. To compare the gravity of any two minerals, simply hold a sample of one in your hand and "heft it," i.e., get a feeling for its weight. Then heft a sample of the other that is approximately the same size. If there is a great difference in gravity, you will detect it easily. It is often sufficient to note whether a mineral's gravity is significantly higher or lower than that of other minerals. Heft each of the specimens in your mineral set. Which ones have a high gravity and a low one compared to the other.

It also is a measure of the density of a mineral. At times it is such a useful property that it is the only way to distinguish some minerals without laboratory or optical techniques. Gold can easily be distinguished from "fool's gold" by specific gravity alone, although there are many other ways. Specific gravity is a unit less measure, because it is derived from the density of the mineral divided by the density of water and thus all units cancel. However, since water's density equals 1 gram per cubic centimeter (at specific conditions), then a mineral's specific gravity would also correspond to a mineral's density as expressed in grams per cubic centimeter.

It also compares the density of a mineral to the density of water. If a mineral has a density of 2, then it is twice as dense as water. If a mineral has a density of 3 then it is three times as dense as water and so forth. However, comparing a mineral to water is not practical and is not really helpful. It is easier to consider what the gravity of a typical mineral is and compare minerals that way.

The Earth's crust, where we are most likely to collect minerals from, is composed mostly of the minerals quartz calcite and feldspar. These minerals have gravities around 2.75 and that is close to the average gravity of the rocks on the outer surface of the Earth's crust. Therefore, the gravity of most rocks that people would pick up and be familiar with will have a gravity of approximately 2.75. To use specific gravity, hold a mineral of unknown gravity in one hand and in the other hand a mineral of known gravity preferably one near the average of 2.75 and of the same size as the unknown mineral; then compare. How does the unknown mineral compare? By convention, comparisons of gravity are split between luster categories: non-metallic and metallic. Non-metallic minerals tend to be of a low density and we naturally expect them to be so. So, when one is surprisingly heavy, it becomes very diagnostic.

By: Louise Kalyn

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This article touched the basics of the topic. I have 2 more resources related to the above. They are specific gravity and specific gravity. Do consider reading them.