How To Do a Specific Gravity Test

What is a Specific Gravity Test?

A specific gravity test compares the density of a material with the density of water. Different minerals often have characteristic specific gravity values, so the test can help identify an unknown stone or distinguish between two minerals that look very similar.

The principle behind the test goes back more than 2,000 years to the Greek mathematician Archimedes. He discovered that an object submerged in water is pushed upwards by a force equal to the weight of the water it displaces. This principle is still used to measure the specific gravity of rocks, minerals and gemstones.

A stone with a specific gravity of 3.0 is approximately three times as dense as water.

Unlike density, specific gravity has no unit. The result is just expressed as a number, for example 2.62 or 3.0.

An Easy Test to Do At Home

A specific gravity test is easy to do at home. You need a reasonably accurate digital scale, a plastic container for water and something to suspend the stone in the water.

First, weigh the dry stone and record the weight in grams.

Next, place the container of water on the scale and reset the weight to zero.

The stone needs to be completely submerged without touching the sides or bottom of the container. A thin wire, thread or netting can be used to support it. When we tested a stone that we believed was howlite, we used paper masking tape.

Whatever you use to hold the stone will affect the weight, so it must be weighed first. Lower the tape, thread or other support into the water to the depth it will reach during the test and record its weight. Do this without the stone attached.

If you use masking tape, you may not be able to reuse it once it gets wet. When replacing it, make sure the new piece is exactly the same length.

Next, secure the stone and lower it fully into the water. It must be as close as possible to the depth of the support when it was weighed without the stone.

For a steadier result, you could suspend the stone from a ruler positioned above the container. It must be supported separately and must not rest on the container or the scale.

Check that no air bubbles are attached to the stone. If there are, gently move the stone or tap the container until the bubbles rise to the surface.

With the stone fully submerged, the scale shows the weight of the water displaced by the stone.

Record the weight, then subtract the weight of the support (the tape without the stone). Now divide the dry weight of the stone by that weight.

For example, if the stone weighs 12 grams, the submerged masking tape weighs 0.1 grams, and the stone and tape together weigh 4.6 grams:

4.6 − 0.1 = 4.5

12 ÷ 4.5 = 2.67

The specific gravity of the stone is therefore approximately 2.67. This can then be compared with known values for different minerals.

Testing Our Howlite Stone

We used this method to establish whether a stone was howlite or magnesite. These two minerals can look very similar, but have a different specific gravity.

Measured values for howlite are generally around 2.5 to 2.6. Magnesite is usually around 3.0, although fine-grained massive magnesite can be around 2.9 to 3.0.

Our stone weighed 42.5 grams. When suspended in the water secured to masking tape, it weighed about 14.8 grams. The weight varied slightly because of movement from holding the tape.

After subtracting the weight of the tape, the corrected weight was 14.7 grams. We repeated the test several times to make sure the results were consistent.

42.5 ÷ 14.7 = 2.89

The specific gravity of our stone was therefore about 2.89. That's significantly higher than howlite and very close to the range for fine-grained magnesite.

Although a specific gravity test alone cannot provide a definitive identification, our result strongly suggests that the stone we believed to be howlite was actually magnesite.

What Can Affect the Result?

For the test to be reasonably accurate, the stone must be completely submerged without touching the container. Air bubbles, movement and differences in how much of the support is submerged can all affect the result.

Porous stones can also absorb water. Turquoise is a good example, particularly lower-grade stones, which tend to be more porous. Natural turquoise stabilised with resin may also give a different result.

If testing a porous stone or one that should not ideally get wet, such as malachite, keep it submerged for as little time as possible. Once removed, dry it thoroughly with a soft cloth.

Some stones should not be put in water at all. These include angelite, selenite and halite. Pyrite and turquoise can be submerged briefly, but should be dried thoroughly immediately afterwards.

Eureka! I Have Found It

There's a well-known story that Archimedes discovered the idea of displacing water while getting into a bath. As he lowered himself into the water, the bath overflowed.

He had been asked to determine whether a gold crown made for King Hiero II had been mixed with cheaper metal. According to the story, Archimedes realised that the amount of water displaced could help reveal the crown's density.

It's claimed that in his excitement, he ran through the streets naked, shouting “Eureka!”, meaning “I have found it”.

The story was recorded by the Roman architect Vitruvius around two hundred years later, so how much of it is true is not known.

Nothing in Archimedes' surviving works mentions the bath, crown or the “Eureka” story.