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Step by Step Specific Gravity Instructions

Specific gravity (SG) is the density of a stone relative to the density of water. The specific gravity of

water is one (1). If a stone has a SG is 3.95, then stone has a density that is 3.95 times greater than

the density of water. SG is determined by dividing the stone's weight in air by the weight of the water

is displaces when it is immersed in water. Because it does not require a polished surface, SG is oneof the best tests for rough stones. You can perform the test in the field and with relatively inexpensive

equipment.

The following directions are for the PrettyRock.com Specific Gravity Travel Kit.

(http://prettyrock.com/php/product-detail.php?q=&v=&item=5803) The kit fits stones up to about 50

ct or approximately 25 x 25 x 15mm and has reasonable accuracy up to .33 ct. You will need the kit,

a calculator, and something to take notes on.

Step 1: Weigh Stone in Air

There is no levitation or magic carpets involved. Instead it is called "weigh in air" to contrast itwith the next step which is “weigh in water."

1. Turn scale on and wait for the reading to go to zero. If it does not go to zero, press the

tare button on the scale. Be sure the mode is set to carats.

2. Put your clean and dry stone on the scale.

3. Record the weight of the stone. We suggest you take each measurement 3 times and

http://prettyrock.com/php/product-detail.php?q=&v=&item=5803

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average your results.

 

Step 2: Weigh Stone in Water 

Make sure your stone is not water soluble such as halite!

1. Start with the scale turn o!. Place the clear shelf over the scale with the shelf's back

edge lining up with the back edge of the circular depression on the scale's weighing

platform.

2. Slip the wire stand holder into the empty water cup. Place the base of the wire standunder the clear shelf, on the scale. The water cup should be on top of the clear shelf.

Place the bowl on the wire stand (might not be needed for larger stones.)

3. This is the fiddly bit. Arrange the wire stand so that it is not touching either the water cup

or the clear shelf. You might need to angle either the wire stand or the water cup. Once

you have done this a few times, you will be able to do this quickly.

4. Carefully fill the water cup with water.

5. Turn the scale on and wait for the reading to go to settle, then press tare to zero the

reading. Check to see that the mode is set to carats.

6. Gently place your stone on the holder in the water so that the stone is completely

submerged and suspended. Tap away any air bubbles. Give the setup and the water

some time to stop moving or shaking.

7. Record the weight of the stone. We suggest you take each measurement 3 times and

average your results.

Step 3: Specific Gravity Formula:

Using the air weight and water weight of the stone, find the specific gravity using the following

formula: AIR WEIGHT / (AIR WEIGHT - WATER WEIGHT)

For example if the air weight is 1.50 carats and the water weight is 1.12 carats:

1.50/(1.50-1.12) = The specific gravity equals 3.95

Specific Gravity Calculator

Enter Air Weight Enter Water Weight SG Results

00.0

Step 4:

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Compare your results with this printable reference chart: Specific Gravity Chart

(/images/specificgravitychart.jpg)

Or plug it into our handy dandy Gem Wizard. (/php/gem-id-database-BETA3.htm)

Pro tips:

Make sure your table is extremely stable! This is critical to getting an accurate reading.

Trying to work on a shaky table or near something mechanical that vibrates (like an air

conditioner!) can make getting a good reading impossible.

Inclusions and impurities in the stone can a!ect the accuracy of the test.

Use distilled water. Minerals in water can a!ect SG.Make sure your stone is clean. Grease can cause air bubbles to stick to your stone.

Get the air weight first, and then the water weight to be sure the stone is dry.

 Avoid any extra friction. The wire stand and bowl must not be touching the water cup.

 Always perform at least three gemological tests when identifying a stone. No one test is

conclusive.

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