ific gravity of all stones, and dividing them into six
groups, by taking a series of standard solutions selected from one or
other of the above, and of known specific gravity, we can judge with
accuracy if any stone is what it is supposed to be, and classify it
correctly by its mere floating or sinking when placed in these liquids.
Beginning then with the pure double nitrate of silver and thallium, this
will isolate the stones of less specific gravity than 4.7963, and taking
the lighter solutions and standardising them, we may get seven solutions
which will isolate the stones as follows:--

A {shows the stones which have} 4.7963
{a specific gravity over}
B " " " 3.70 and under 4.7963
C " " " 3.50 " 3.70
D " " " 3.00 " 3.50
E " " " 2.50 " 3.00
F " " " 2.00 " 2.50
G " " -- -- under 2.00

Therefore each liquid will isolate the stones in its own group by
compelling them to float on its surface; commencing with the heaviest
and giving to the groups the same letters as the liquids, it is seen
that--

_Group_ A.--Isolates gems with a specific gravity of 4.7963 and over
4.70; in this group is placed zircon, with a specific gravity of from
4.70 to 4.88.

_Group_ B.--Stones whose specific gravity lies between 3.70 and under
4.7963.

Garnets, many varieties. See Group D below.
Almandine 4.11 and occasionally to 4.25
Ruby 4.073 " 4.080
Sapphire 4.049 " 4.060
Corundum 3.90 " 4.16
Cape Ruby 3.861
Demantoid 3.815
Staurolite 3.735
Malachite 3.710 and occasionally to 3.996

_Group_ C.--Stones whose specific gravity lies between 3.50 and under
3.70.

Pyrope (average) 3.682
Chrysoberyl 3.689 and occasionally to 3.752
Spinel 3.614 " 3.654
Kyanite 3.609 " 3.688
Hessonite 3.603 " 3.651
Diamond 3.502 " 3.564
Topaz 3.500 " 3.520

_Group_ D.--Stones whose specific gravity lies between 3 and under 3.50.

Rhodonite 3.413 and occasionally to 3.617
Garnets 3.400 " 4.500
Epidote 3.360 " 3.480
Sphene 3.348 and occasionally to 3.420
Idocrase