Specific Gravity ( SG )
The weight of a gemstone in air compared to an equal volume of pure water at 4 degrees Celsius.
Weight in Air
Weight in Air - Weight in Water
1 cc of water at 4 degrees Celsius = One gram.
Methods for determining Specific Gravity
- Displacement Method.
- Hydrostatic Method.
- Heavy Liquids.
Suitable only for determining the Specific Gravity of large objects.
- Weigh the object in air.
- Fill a Eureka Can with water at 4 degrees Celsius until the water starts to flow from the spout.
- Allow the water to stabilize and the overflow to cease.
- Place a graduated vessel (marked in cc's) under the spout.
- Carefully lower the object to be tested into the water.
- Convert the water displaced (measured in cc's and representing the volume of the object) into grams.
- Use the S.G formula.
- Weigh the stone in air using a precision scale.
- Adapt a beam balance scale by placing a four legged stand over the left hand pan so that the swing is unaffected.
- Place on the stand a beaker of water at 4 degrees Celsius.
- 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.
- 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.
- Weigh the stone in the water.
- Use the S.G Formula.
Varying S.G's can be achieved by diluting Methylene Iodide or Bromoform with either Benzyl Benzoate or Monobromonapthalene.
- Clean the stone to be tested thoroughly.
- Place the stone in one of the Heavy Liquids.
- 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
- The Hydrostatic Method is only good for larger stones, under 3.00 carats errors are evident.
- The Hydrostatic Method is time consuming.
- Heavy liquids can damage porous stones.
- Can only be used for unmounted stones.
- Heavy liquids are unpleasant to deal with and in some cases
- can be dangerous.
- Useful when dealing with unpolished stones or stones of high Refractive Index.
- Easy to use (Heavy Liquids).
- Dissolve 10 teaspoons of salt into 1/2 pint of water producing an S.G of 1.12 to 1.14
Amber and Copal Resin will float while all other Amber imitations will sink.