The
identification tests used on the diamonds are the following:
insolubility in hydrofluoric acid, hardness, specific gravity, index
of refraction, and burning in an atmosphere of oxygen. The power of a
transparent medium to refract a ray of light is expressed by a certain
number called index of refraction. The phenomenon of dispersion shows
that this number is different for rays of colors, or wave lengths. It
is generally greater in proportion to the density of the medium.
A
diamond is insoluble in hydrofluoric acid, although this dissolves most
other substances. The hardness of a diamond is tested by scratching on
carborundum. The specific gravity of the diamond is usually tested in
methylene iodide which has a specific gravity of 3.33 at 150 C. and very close to that of the diamond which is 3.51. Diamonds burn in oxygen at 800 degrees, Centigrade.
In
the burning test, the specimen is placed in a platinum boat, which in
turn is placed inside a silicon tube and heated to 800 degrees,
Centigrade, while oxygen is passed into one end of the tube and out of
the other and through a Geissler tube containing potassium hydroxide.
If the specimen is a diamond it will burn, forming carbon dioxide which
is absorbed in the Geissler tube. A pure diamond leaves no ash in the
platinum boat, hence any ash remaining in the boat is positive proof
of impurities in the diamond. These impurities are generally metallic
oxides. They can also be detected by the color of the diamond—a pure
diamond is transparent, while impure diamonds are tinted various
colors. A number of the smaller synthetic diamonds
*39
