Advantages
- The major advantage is that an exact specific gravity (SG) figure can be arrived at.
- There is no limitation on the specific gravity (SG) of the stones that can be calculated. Unlike heavy liquids, high SG stones can be tested just as accurately as low SG stones.
Disadvantages
The accuracy of this method depends on several things:
- The accuracy of the balance. The balance should be able to weigh stones to 0.01 carat. If not, degree of error will be too large.
- The size of the stone is of great importance. With a ruby weighing 2 carats, an error of just 0.01 carat in the water weighing will lead to an error of 0.08 in the final SG determination, which is qute serious. It is generally agreed that even with a very accurate balance, this method should not be used with stones of less than one carat.
- The expertise and skill of the person making the measurements is also important. There are possibilities for error in both the weighing and the calculations. Things such as surface tension and air bubbles in the water can also affect the results.
- The hydrostatic method is rather time consuming and even someone with a great deal of experience usually needs several minutes to calculate the SG of a single stone.
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