Specific Gravity ( SG )

Definition:

The weight of a gemstone in air compared to an equal volume of pure water at 4 degrees Celsius.

1 cc of water at 4 degrees Celsius = One gram.

Methods for determining Specific Gravity

1. Displacement Method.
2. Hydrostatic Method.
3. Heavy Liquids.

Displacement Method

Suitable only for determining the Specific Gravity of large objects.

1. Weigh the object in air.
2. Fill a Eureka Can with water at 4 degrees Celsius until the water starts to flow from the spout.
3. Allow the water to stabilize and the overflow to cease.
4. Place a graduated vessel (marked in cc's) under the spout.
5. Carefully lower the object to be tested into the water.
6. Convert the water displaced (measured in cc's and representing the volume of the object) into grams.
7. Use the S.G formula.

Hydrostatic Method

Procedure:

1. Weigh the stone in air using a precision scale.
2. Adapt a beam balance scale by placing a four legged stand over the left hand pan so that the swing is unaffected.
3. Place on the stand a beaker of water at 4 degrees Celsius.
4. Using non-capillary wire, make a cage and attach the wire to the weigh pan hanger so that the cage is immersed in the water and stays submerged during each full swing without touching the sides.
5. Attach a counterpoise to the other weigh pan, shorter than the other side since it will have experienced weight loss by being submerged in the water.
6. Weigh the stone in the water.
7. Use the S.G Formula.

Heavy Liquids

Recommended liquids:

Varying S.G's can be achieved by diluting Methylene Iodide or Bromoform with either Benzyl Benzoate or Monobromonapthalene.

Procedure:

1. Clean the stone to be tested thoroughly.
2. Place the stone in one of the Heavy Liquids.
3. Observe one of the following reactions:

• If the stone floats - the stone has an S.G lower than the liquid.
• If the stone sinks - the stone has an S.G greater than the liquid.
• If the stone becomes freely suspended in the liquid - the S.G of the stone is equal to that of the liquid.

Disadvantages of S.G Determination

1. The Hydrostatic Method is only good for larger stones, under 3.00 carats errors are evident.
2. The Hydrostatic Method is time consuming.
3. Heavy liquids can damage porous stones.
4. Can only be used for unmounted stones.
5. Heavy liquids are unpleasant to deal with and in some cases
6. can be dangerous.

Advantages

1. Useful when dealing with unpolished stones or stones of high Refractive Index.
2. Easy to use (Heavy Liquids).

Amber Test

Procedure :

1. Dissolve 10 teaspoons of salt into 1/2 pint of water producing an S.G of 1.12 to 1.14

Observations :

Amber and Copal Resin will float while all other Amber imitations will sink.