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