BOART, CARBONADO, AND GRAPHITE
The black inclusions in some transparent diamonds consist of
graphite. On crushing a clear diamond showing such spots and
heating in oxygen to a temperature well below the point at which
diamond begins to burn, Moissan found that the grey tint of the
powder disappeared, no black spots being seen under the microscope.
There also occur what may be considered intermediate forms between
the well-crystallised diamond and graphite. These are “boart” and
“carbonado.” Boart is an imperfectly crystallised diamond, having
no clear portions, and therefore useless for gems. Shot boart is
frequently found in spherical globules, and may be of all colours.
Ordinary boart is so hard that it is used in rock-drilling,
and when crushed it is employed for cutting and polishing other
stones. Carbonado is the Brazilian term for a still less perfectly
crystallised form of carbon. It is equally hard, and occurs in
porous masses and in massive black pebbles, sometimes weighing two
or more ounces.
The ash left after burning a diamond invariably contains iron as
its chief constituent; and the most common colours of diamonds,
when not perfectly pellucid, show various shades of brown and
yellow, from the palest “off colour” to almost black. These
variations give support to the theory advanced by Moissan that
the diamond has separated from molten iron--a theory of which I
shall say more presently--and also explain how it happens that
stones from different mines, and even from different parts of the
same mine, differ from each other. Further confirmation is given
by the fact that the country round Kimberley is remarkable for
its ferruginous character, and iron-saturated soil is popularly
regarded as one of the indications of the near presence of diamonds.
GRAPHITE
Intermediate between soft carbon and diamond come the graphites.
The name graphite is given to a variety of carbon, generally
crystalline, which in an oxidising mixture of chlorate of potassium
and nitric acid forms graphitic oxide. This varies in colour
from green to brown or yellow, or it is almost without colour,
according to the completeness of the reaction. Graphites are of
varying densities, from 2·0 to 3·0, and generally of crystalline
aspect. Graphite and diamond pass insensibly into one another. Hard
graphite and soft diamond are near the same specific gravity. The
difference appears to be one of pressure at the time of formation.
Some forms of graphite exhibit the remarkable property by which
it is possible to ascertain approximately the temperature at
which they were formed, or to which they have subsequently been
exposed. Sprouting graphite is a form, frequently met with in
nature, which on moderate heating swells up to a bulky, very
light mass of amorphous carbon. Moissan has found it in blue
ground from Kimberley; my own results verify his. When obtained by
simple elevation of temperature in the arc or the electric furnace
graphites do not sprout; but when they are formed by dissolving
carbon in a metal at a high temperature and then allowing the
graphite to separate out on cooling, the sprouting variety appears.
The phenomenon of sprouting is easily shown. If a few grains are
placed in a test-tube and heated to about 170° C., the grains
increase enormously in bulk and fill the tube with a light form of
amorphous carbon.
The resistance of a graphite to oxidising agents is greater the
higher the temperature to which it has previously been exposed.
Graphites which are easily attacked by a mixture of fuming nitric
acid and potassium chlorate are rendered more resistant by strong
heat in the electric furnace.
I have already signified that there are various degrees of
refractoriness to chemical reagents among the different forms of
graphite. Some dissolve in strong nitric acid; other forms of
graphite require a mixture of highly concentrated nitric acid and
potassium chlorate to attack them, and even with this intensely
powerful agent some graphites resist longer than others. M. Moissan
has shown that the power of resistance to nitric acid and potassium
chlorate is in proportion to the temperature at which the graphite
was formed, and with tolerable certainty we can estimate this
temperature by the resistance of the specimen of graphite to this
reagent.
CRYSTALLISATION
The diamond belongs to the isometric system of crystallography; the
prevailing form is octahedral. It frequently occurs with curved
faces and edges. Twin crystals (macles) are not uncommon. Diamond
crystals are generally perfect on all sides. They seldom show
irregular sides or faces by which they were attached to a support,
as do artificial crystals of chemical salts; another proof that the
diamond must have crystallised from a dense liquid.
The accompanying illustration (Fig. 14) shows some of the various
crystalline forms of native diamonds.
[Illustration: FIG. 14. CRYSTALLINE FORMS OF NATIVE DIAMONDS.
To face p. 86.]
No. 1. Diamond in the form of a hexakis-octahedron (the forty-eight
scalenohedron), or a solid figure contained by forty-eight scalene
triangles. According to Professor Maskelyne, this occurs as a
self-existent form only in the diamond.
No. 2. Diamond in the form of a hexakis-octahedron and
octahedron. From Sudafrika.
No. 3. Diamond in the form of octahedron with intersections.
No. 4. Diamond from Brazil.
No. 5. Diamond from Kimberley.
No. 6. Diamond from Brazil.
No, 7. A macle or twin crystal, showing its formation from an
octahedron with curved edges.
* * * * *
Some crystals of diamonds have their surfaces beautifully marked
with equilateral triangles, interlaced and of varying sizes
(Fig. 15). Under the microscope these markings appear as hollow
depressions sharply cut out of the surrounding surface, and
these depressions were supposed by Gustav Rose to indicate the
probability that the diamonds had at some previous time been
exposed to incipient combustion. Rose pointed out that similar
triangular striations appeared on the surfaces of diamonds burnt
before the blowpipe. This experiment I have repeated on a clear
diamond, and I have satisfied myself that during combustion
before the blowpipe, in the field of a microscope, the surface is
etched with triangular markings different in character from those
naturally on crystals (Fig. 16). The artificial striæ are very
irregular, much smaller, and massed closer together, looking as
if the diamond during combustion flaked away in triangular chips,
while the markings natural to crystals appear as if produced by
the crystallising force as they were being built up. Many crystals
of chemical compounds appear striated from both these causes.
Geometrical markings can be produced by eroding the surface of a
crystal of alum with water, and they also occur naturally during
crystallisation.
[Illustration: FIG. 15. TRIANGULAR MARKINGS ON NATURAL FACE OF A
DIAMOND CRYSTAL.]
[Illustration: FIG. 16. TRIANGULAR MARKINGS ARTIFICIALLY PRODUCED
ON A DIAMOND CRYSTAL.
To face page 88.]