It is assumed that the jeweler will weigh in carats, and that his balance is sensitive to .01 carat. With such a balance, and a specific gravity bottle (which any scientific supply house will furnish for less than $1) results sufficiently accurate for the determination of precious stones may be had if one is careful to exclude air bubbles from the bottle, and to wipe the outside of the bottle perfectly dry before each weighing. The bottle should never be held in the warm hands, or it will act like a thermometer and expand the water up the narrow tube in the stopper, thus leading to error. A handkerchief may be used to grasp the bottle.
Table of Specific Gravities of the Principal Gem Materials
| Beryl (Emerald) | 2.74 | |
| Chrysoberyl (Alexandrite) | 3.73 | |
| Corundum (Ruby, sapphire, "Oriental topaz") | 4.03 | |
| Diamond | 3.52 | |
| Garnet | (Pyrope) | 3.78 |
| " | (Hessonite) | 3.61 |
| " | (Demantoid, known in the trade as "Olivine") | 3.84 |
| " | (Almandite) | 4.05 |
| Opal | 2.15 | |
| Peridot | 3.40 | |
| Quartz (Amethyst, common topaz) | 2.66 | |
| Spinel (Rubicelle, Balas ruby) | 3.60 | |
| Spodumene (Kunzite) | 3.18 | |
| Topaz (precious) | 3.53 | |
| Tourmaline | 3.10 | |
| Turquoise | 2.82 | |
| Zircon, | lighter variety | 4.20 |
| " | heavier variety | 4.69 |
For a more complete and scientific discussion of specific gravity determination see Gem-Stones, by G. F. Herbert-Smith, Chapter VIII., pp. 63-77; or see, A Handbook of Precious Stones, by M. D. Rothschild, pp. 21-27, for an excellent account with illustrations; or see any physics text-book.
LESSON VI
SPECIFIC GRAVITY DETERMINATIONS
Weighing a Gem in Water. In the previous lesson it was seen that the identity of a precious stone may be found by determining its specific gravity, which is a number that tells how much heavier the material is than a like volume of water. It was not explained, however, how one would proceed to get the specific gravity of a stone too large to go in the neck of a specific gravity bottle. In the latter case we resort to another method of finding how much a like volume of water weighs. If the stone, instead of being dropped into a perfectly full bottle of water (which then overflows), be dropped into a partly filled glass or small beaker of water, just as much water will be displaced as though the vessel were full, and it will be displaced upward as before, for lack of any other place to go. Consequently its weight will tend to buoy up or float the stone by trying to get back under it, and the stone when in water will weigh less than when in air. Anyone who has ever pulled up a small anchor when out fishing from a boat will recognize at once that this is the case, and that as the anchor emerges from the water it seems to suddenly grow heavier. Not only does the stone weigh less when in the water, but it weighs exactly as much less as the weight of the water that was displaced by the stone (which has a volume equal to the volume of the stone). If we weigh a stone first in the air, as usual, and then in water (where it weighs less), and then subtract the weight in water from the weight in air we will have the loss of weight in water, and this equals the weight of an equal volume of water, which is precisely what we got by our bottle method.
We now need only divide the weight in air by the loss of weight in water, and we shall have the specific gravity of the stone.