some
of it contains so much copper that it is crushed and smelted along with
the regular ore. The gem material varies in color from pure opaque
enamel white in a compact granular variety to cream, yellowish, pink,
and purplish in more translucent material. There are also dark to black
streaks and mottlings through the massive datolite of different colors.
The colors in the datolite are due to the presence of native copper
more or less finely divided. Apparently the purplish tints are due to
extremely finely divided copper, and the pink and yellow tints are
caused by larger scales and particles of copper visible to the naked
eye or through a hand glass. Some-of the specimens are attached to trap
rock, much epidotized in places with native copper. These specimens
furnish attractive material for small ornaments and some are suitable
for gems. The datolite is cut into stones for scarf pins, cuff buttons,
watch charms, etc. They are sold by jewelers in the copper-producing
section of Michigan and also more widely in the gem markets.
DIAMOND.
UNITED STATES.
Arkansas.—A
pamphlet issued by the Arkansas Diamond Company of Little Rock,
containing the statements of the original investors, the mining
engineer, and the geologist of the company will prove of interest to
many people. An article giving an outline of the discovery,
developments, and possibilities of the deposit has been written also by
the company's mining engineer, John T. Fuller." Mr. Fuller calls
attention to the fact that the term "blue ground" is a misnomer and is
little understood by the average prospector. The "blue" refers to the
bluish-green color of the unweathered serpentinized peridotite
encountered below the zone of yellow weathered material at the surface.
On exposure to weather, the "blue," which is really in the form of
rock, disintegrates to a grayish-green or yellowish-green friable mass
which is more properly a 'ground." The peridotite of the Arkansas
Diamond Company mine is essentially similar to that of the South
African diamond mines. Since the peridotite in the latter mines varies
greatly in appearance, not only in different mines but often in
different parts of the same mine, it is not expected that a close
comparison between details can be made. The occurrence of the Arkansas
peridotite in the form of a pipe, the manner in which it weathers, the
presence of unaltered portions called "hardibank," and of minerals
commonly found associated with diamonds are points of similarity.
On
over half of the area of the peridotite outcrop the rock has weathered
to depths varying from 20 to 60 feet, so that it will require but a
minimum of blasting to mine. The material that can be thus removed is
estimated at 1,500,000 cubic yards. In the work of testing the deposit
the yield of diamonds was promising and amounted to 540 stones, of
which 505 weighed together 217 carats. Three cut stones were found to
be brilliant and were valued at from $60 to $175 per carat, with an
average value of $104 per carat. A parcel of rough unsorted stones from
the mine will be easily worth $10 per carat. The only way to obtain a
true idea of the value or possibilities
a Eng. and Min. Jour., January 16,1909.
