PHYSICAL PROPERTIES, ETC.
Before explaining, or rather attempting to explain, the phenomena
of some of the prominent physical properties of the mineral, it is
proper that we should give a description of its forms and its natural
appearance as it is taken from the mines, so that our reader may become
more familiar with the subject. We will not, however, venture very far
among the dry details of crystallography, even if it be a subject of
great interest to the student in science. The stone which so readily
attracts the eye by its dazzling splendors after it has received a
definite form and polish from art, is seldom attractive to the view
unless it occurs in a rare and perfect form of crystallization. Even
then, in this primitive state it exhibits none of the rainbow play
of color which makes the stone so celebrated and so beautiful. In
reality, in these rare conditions it is seldom if ever so lustrous and
pleasing to the sight as crystals of many other minerals. In general,
the diamond is so obscure in its attractions that practised eyes are
required for its search.
Recently the distinguished savant Von Tschudi, in visiting some of
the diamond mines of Brazil, was unable to distinguish readily the
gems as they lay in the washed cascalho, while the trained eyes of the
negro miners picked them out with ease. It has been stated that the
diamonds are always or nearly always covered with an earthy crust of
various hues, especially greenish or yellowish, which is hardly the
fact; for what appears to be a crust is often caused by the salient
edges of the laminæ, among which a little earthy or coloring matter
has been introduced. These extraneous colors generally disappear when
the surface of the stone is removed; and, in fact, the degree of their
intensity is very much modified when the rough gem is placed in alcohol
or in any fluid of high refraction.
The dull, whitish appearance seen in the natural diamonds is also
produced by the action of fire, which raises the edges of the laminæ,
producing a faint milky aspect.
The primitive form of the mineral is the octahedron, and many irregular
masses may be reduced by cleavage to that of a double pyramid.
These octahedral crystals are sometimes as perfect in outline as a
mathematical model, with clean-cut angles and smooth faces. In some
specimens the edges may be truncated, that is, as if they had been
flattened or ground off by mechanical means. Generally, however, the
crystals are of the form of the octahedrons with rounded faces. The
dodecahedrons, with their twelve faces, and the cubes, with their
four sides, may also be reduced by cleavage to the primitive form of
the double pyramid. Sometimes two crystals are united, forming what
are called hemitrope or twin crystals. Then, again, a number may be
grouped together, assuming on the whole a globular-like mass. But they
are decidedly different from the globular, which in their form of
crystallization radiate from the centre of the crystal.
The variety of diamond called boart, or bort, deriving its name from
the supposed abortive attempt of nature to form a perfect crystal,
is also quite deficient in cleavage, or its laminæ are so irregular
as to render splitting quite impossible and the cutting of the stone
equally so. The transparency of these forms is also affected by the
arrangement of crystallization; hence they are generally crushed into
powder for polishing material or used for various purposes in the
arts. The specific gravity of these varieties seems to be influenced
by the manner of crystallization. For instances, we find that the fine
transparent crystals have a specific gravity of 3.55 (water being
considered the standard as 1), while the bort is somewhat less; and the
massive variety called the carbonado varies from 3 to 3.4, according to
the amount of earthy matter it may contain. It has also been asserted
that the blue, the green, the orange, and the red varieties are heavier
than the white. The phenomena of electricity observed in the diamond
are not remarkable, and are inferior to most of the gems. Some of the
precious stones when excited retain their electrical properties for
hours or even days, but the diamond loses it almost immediately. It
exhibits vitreous electricity when rubbed.
Much has been said and written concerning the artificial
phosphorescence exhibited by the diamond when removed to a dark room
after having been exposed for a short time to the sun’s rays. We are
not able to verify this statement, and feel inclined to doubt its
correctness, although we have been assured by experimenters of the fact.
One of the most remarkable properties of the diamond is its extreme
hardness, in which it far exceeds all known substances in the mineral
kingdom. This peculiarity is due to the substance itself, but appears
to be modified by its color and its form of crystallization like some
other minerals.
The more perfectly the crystal is formed, the easier its laminæ become
detached, and the softer the substance appears to be. In the globular
forms, which are quite deficient in cleavage planes, the hardness
is excessive, and often resists the most determined efforts of the
lapidary. Even in fine crystals we shall find that certain angles are
harder than others; and we may observe the same relative degree among
crystals of other minerals, like those of the topaz. In the large
transparent diamonds of irregular form, spots of excessive hardness are
often found. These are called by the lapidaries “knots,” and appear to
be due to a change in the process of crystallization. The coloring
matter, or the mode of its formation, seems to affect the degree of
hardness in many minerals; and in respect to the diamond, the rare
crystallized black form is harder than the limpid or lighter colored.
Some years ago a black diamond from Borneo was placed in the hand
of Gallais the lapidary, to be experimented upon at the expense of
the French Institute. The chief object of the test was to ascertain
the relative degree of hardness in comparison with some of the other
varieties of diamond. In this trial the lapidary wore out his steel
wheel and a large quantity of ordinary diamond dust without making
the least impression on the surface of the black diamond. Although
heavily loaded with weights, it lost none of its roughness, and was
heated almost to whiteness by the friction of the wheel, which revolved
with great velocity. During the period of this extreme velocity it is
reported that a shower of sparks was emitted; but how shall we account
for this scintillation, when the ordinary transparent diamond does not
give forth sparks when struck by steel?
The carbonado, which is amorphous and without cleavage, is also
extremely hard. The term “adamas,” which the ancients bestowed upon it
as denoting an invincible infrangibility, is not quite appropriate; for
although it is far superior in hardness to all other known substances,
it is in reality very fragile. And in the power to resist the effect
of shock it is also inferior to some of the other gems, and especially
the sapphire. Therefore several mineralogists have thought that the
ancients really applied the term to steel or to some of the varieties
of corundum, like the ruby and the sapphire, and not the diamond.
It is curious that this property should be ascribed erroneously to
the diamond for so many ages, when a trifling experiment would have
disclosed the real condition of things. In the days of the poet
Lucretius the gem was believed to be able to resist violent blows.
----“adamantina saxa
Prima acie constant, ictus contemnere sueta.”
Pliny entertained the same idea, and also that its infrangibility
could be overcome only by first steeping it in goats’ blood. Even in
mediæval times Ben Mansur, the Persian mineralogist, gravely states
that a diamond laid upon the anvil and struck by a hammer would not
be broken, but would be driven by the violence of the blow into the
substance of the anvil. This stupid but wide-spread idea has prevailed
even in modern times; and many a gem has been sacrificed by the
ignorant in testing the character of the stone. The brittleness of the
gem is partly due to its singular cleavage, which in regular crystals
is so perfect and uniform as to permit the lapidary to remove the
laminæ so as to entirely demolish the structure of the crystal. But
when once accomplished, no artisan, however skilful, can replace them
again. The facility with which the stone may be separated was known in
ancient times among the Hindoos, and probably in Europe as early as the
sixteenth century, as De Boot knew of a physician who could divide the
diamond into thin scales like a piece of talc; but it was forgotten
until Wollaston not many years ago stumbled upon the secret of cleavage
and made it known to modern science.
The real charm and value of the diamond lie in its remarkable
brilliancy, and in the wonderful prismatic display of the bright and
beautiful colors, which are constantly fugitive, but perpetually
returning, as the learned Abbé Haüy elegantly expresses it. When a
ray of light is reflected from the surface of a body, a particular
impression is conveyed to the eye, which we may properly term the
eclat. This impression is often so decided and so varied in its
effects, that we are able to distinguish certain substances at a
glance; and the reflection from the diamond exhibits a peculiarity
which is seen only in a very few substances. This is known as the
adamantine flash, and none of the gems display it to any marked degree
except the rare zircon. We witness the perfection of this property in
the black and opaque but crystallized diamond, when faceted by art; and
also in some few minerals of which we shall soon make mention. When
the rays of light are refracted, after passing through the transparent
diamond after it has been cut in a certain manner, and its facets
are arranged in an exact relation to each other, then we obtain
the remarkable exhibition of color which is known as the prismatic
display. This singular property is seen in perfection, or even to any
considerable degree, only in the diamond, among all the gems thus
far known. But art, however, has succeeded in imitating it in one
of her productions of glass, and so admirably, that under favorable
circumstances it is quite impossible for the eye alone to distinguish
the artificial from the real gem. Some of the theories relating to the
causes of these phenomena we will discuss hereafter, and at the present
will only say that it is to modern science the diamond owes the full
development of its latent beauty; and that the result was not attained
until Newton demonstrated the laws that govern the refraction of light.
It is only in the brilliant and rose-cut forms, or their modifications,
when made with mathematical precision, that the brilliancy and beauty
of the stone is displayed in perfection. The ancients, therefore, were
not acquainted with the full splendors of the gem. For, being ignorant
of the laws of refraction of light, they polished the stone chiefly
with the view of preserving its greatest weight; and, at the same time,
producing perfect transparency. Hence most of the specimens of ancient
and barbaric art are rudely cut, and therefore do not exhibit the
degree of beauty which is latent in the mineral. This is also one of
the reasons why the luxurious Romans preferred the opal to the diamond,
since the polished, or even the rude specimens of opal exhibited their
glorious reflections of wondrous hues, both by day and in artificial
light by night; while the diamond, with its natural or polished faces,
gave forth no prismatic display in the daylight, and but a slight
degree comparatively in artificial light at night.
Whence arises this remarkable brilliancy, and to what particular cause
is the property due? This inquiry has afforded a fruitful theme of
speculation among philosophers, but at the present time we are content
to say that the refractive power of the gem is due to the nature of its
substance. This is somewhat indefinite, it is true, but what else can
we say?
Under the general belief that the harder the gem the higher its
refractive powers would be, it has been maintained that the brilliancy
of the diamond arose from the simple property of its excessive
hardness. Investigation, however, does not sustain this widespread
view. Hardness, indeed, may have considerable relation with the
arrangement and form of the molecules composing the gem, for in the
same crystal it is not uniform,--some faces and angles being harder
than others,--but it does not determine the degree of brilliancy. To
strengthen this statement we will take for instances the soft minerals,
crocroisite, the chromate of lead; the Greenockite, the sulphuret of
cadmium; and the octahedrite, the oxide of titanium, which exceed even
the diamond in brilliancy. There are also other decided examples among
the transparent minerals to sustain this view; the most remarkable
of which perhaps may be found in the zircon, a gem which is soft
as quartz; yet it ranks next to the diamond in brightness, and far
surpasses in eclat every other gem, even the sapphire, which is next
to the diamond in hardness. Density does not seem to have anything to
do with the determination of the refractive power of gems, for the
garnet, spinel, sapphire, and zircon are much heavier than the diamond,
and are yet far inferior in brilliancy. The topaz is exactly of the
same specific gravity as the diamond (3.55), but nevertheless its
refractive powers have but little more than one half the energy of the
diamond. The relative brilliancy of the diamond to that of the purest
limpid quartz is 8 to 3; but the relative density is only as 4 to 3.
All diamonds do not exhibit the same degree of brilliancy, because
they do not possess alike the same quality of purity or perfection of
crystallization.
We often observe among the minerals that the most perfect specimens
are found of a diminutive size; and we shall also find that the finest
and purest types of the diamond occur in stones of little weight. The
larger crystals, or amorphous masses, seem to be wanting in purity
and brightness as compared with the lesser; and this peculiarity may
be observed well marked in some of the other gems. Here, then, we may
find material for the argument that the degree of brilliancy is in a
measure due to the perfection of the crystallization of the stone;
and, therefore, the larger and coarser the laminæ of the crystal the
less will be its brightness. One thing, however, is certain; that the
most brilliant gems are obtained from stones of no great weight, and
which also seem, from their form, to indicate a nodular arrangement of
particles in their formation; or, in other words, a certain concentric
manner of crystallization. This form of deposition is not peculiar
to the diamond, but is clearly shown in the sapphires, topazes,
chrysoberyls, tourmalines; and the finest specimens of these gems are
cut from these nodular forms. We think we are correct in stating that
the greatest brilliancy and the most beautiful prismatic display may
be observed in diamonds of less than ten karats in weight. In fact,
the diminution of brilliancy in the gem, when above twenty karats,
is easily discerned by the eye alone, as compared with the vivid and
adamantine flash of a pure and perfect four or eight karat stone. The
same peculiarity may be observed in the little globular masses of the
chrysoberyl, which are seldom larger than a pea in size, but which,
when cut, exhibit flashes of fire which are only equalled or excelled
by the diamond, or the rarer zircon. We can hardly realize that the
little rounded pebbles of white topaz, known as _gouttes d’eau_, “drops
of water,” will yield gems of such lustre as to be often exhibited,
and even sold for the diamond. Yet the larger irregular masses, or
finely crystallized specimens of the same mineral, do not afford gems
of unusual brilliancy. In these instances we may affirm that the form
or mode of crystallization has something to do with the degree of
brilliancy.
The prismatic play of color which this gem alone possesses to any
considerable degree constitutes its chief charm, and its cause has
been a matter of earnest study among opticians. A plausible theory has
lately been advanced by an English philosopher that the colored rays
are produced by the relation of the high refractive to its very low
dispersive power. For instance, this refractive power in the diamond,
or, in other words, its property of bending a ray of light falling
obliquely upon its surface, is 2.439, while that of water is only
1.336, and that of glass 1.500. But its power of dispersing a ray of
white light, or, in other words, of separating it into its compound
colors in reference to its refractive power, is only 0.038, while
that of glass is 0.052. Hence it is surmised that this inferiority
of dispersive power is required for the production of the splendid
colored reflections which constitute the glory of the gem. It is also
maintained that this high refractive power separates the red and the
blue rays more than a high dispersive power would in other transparent
bodies, and to such degree as to allow each color of the spectrum its
full force. As example, the zircon, with its inferior reflections, is
offered, its refraction being 1.99 on the established scale, while its
dispersive power is as high as 0.044. The relations of the spinel are
also as 1.81 to 0.040, and neither does the gem display the rainbow
hues. This theory is certainly ingenious, and if correct the test
may be applied to other transparent minerals possessing similar
relations. We may, therefore, expect the white garnet to exhibit the
property of prismatic display, as it has a refractive power of 1.81 and
a dispersive power of 0.033. But, unfortunately, perfectly pure and
transparent white garnets are unknown, and we must therefore turn to
other minerals for comparison.
To the white tourmaline, then, we will apply the test, since this
mineral has a refractive power of 1.66, with a dispersive power of
only 0.028. Here, then, we have nearly the same relation as observed
in the diamond; and, if the theory be correct, we may reasonably
expect the exhibition of the same phenomena. But, upon examination of
several perfectly white and transparent tourmalines from Mt. Mica,
cut into regular brilliants, we have failed to detect an increase of
prismatic display, or even discover any evidence to lend support to
the plausibility of the hypothesis. We, therefore, reluctantly turn to
other arguments for a solution of this most interesting problem.
The snow-white diamond displays the rainbow hues in the greatest
perfection; and this is the reason why this quality is sought for in
preference to the light buff or deeper yellow, which are in reality
more brilliant. The deeper the hue of the gem, the less becomes the
prismatic display; and when the diamond becomes of deep and decided
hue, the colored reflections cease altogether. It is somewhat
singular that the colored gems are generally more brilliant than the
pure white, that is, if the color is not so deep as to affect the
transparency of the stone. For examples, we shall find that the white
sapphire has an index of refraction equal to 1.768, while the blue has
1.794, and the red 1.779. The refractive of the white topaz is 1.610,
while the yellow is 1.632.
The brilliancy and rainbow play of the diamond is not so apparent by
daylight as by certain kinds of artificial light, when all its latent
beauties are called forth as if by magic. The light of the camp-fire in
the obscurity of night produces a marvellous effect upon the polished
stone; and it is no wonder that the savage heart of the Russian
General, Suvaroff, was fascinated by the vivid gleams of his treasured
diamonds when viewed at night in the flickering beams of his bivouac
fire. It may seem singular that the brilliant white light of gas does
not display the qualities of the diamond as the duller flame of the wax
candle. The secret lies, perhaps, in the difference in their spectra.
Nevertheless, there is a great difference in their effects upon the
gem, and it is a fact that the wax candle far exceeds the gaslight in
calling forth the latent splendors of the gem. Therefore, we can assert
that the brilliancy of toilets where the diamond is much worn depends
greatly upon the manner of illuminating the apartment.
We now come to another interesting problem in the study of the nature
of the diamond. We refer to the various colors of the gem. As we have
maintained that the mineral is of vegetable origin we may be expected
to explain the phenomena of its color upon this hypothesis, and also
account for the various changes of the gem when exposed to the effects
of heat or the fire test. But we must admit with candor that our views
concerning this physical property are decidedly unsatisfactory, and
shall refer the reader to one of the chapters in our treatise on the
Tourmaline, in which are grouped some of the theories relating to the
subject. In fact, we may repeat the remarks of Huyghens, who said at
the end of the seventeenth century: “In spite of the labors of Newton,
no one has yet fully discovered the cause of the color of bodies.”
“We must, then,” says M. Babinet, “admire, without penetrating their
secret, the unparalleled red of the Oriental ruby, the pure yellow of
the topaz, the unmingled greenness of the emerald, the soft blue of the
sapphire, and the rich violet of the amethyst. This is not the only
thing the discovery of which we shall leave to posterity.”
The color-suite of the mineral is much more extensive and varied than
has been generally admitted by mineralogists. We are led to infer from
their works that white is the prevailing hue of the gem; but Beudant
declares that perfect limpidity and whiteness is rare comparatively,
and that the stone is generally affected with yellowish or brownish
tints. But what becomes of the vast numbers of these clouded or tinged
and inferior gems, if the mines yield so many of them in comparison
with the snow-white? Are they consumed in polishing others, or
expended in the arts, or have the lapidaries secret processes by which
these objectionable tints are expunged from the stone?
Barbot, the French jeweller, declares that, by means of certain
particular and energetic agents, aided by a proper degree of heat, he
is able to remove the greens of all shades, the light-red, and the
yellow, when the coloring matter is superficial, or even situated
between the external laminæ. We are inclined to believe Barbot in this
particular reference, especially as he admits that he is unable to
change much the deep-yellow, the brown, and the smoky-tinted stones.
Of the yellow tints, the diamond affords the most beautiful examples,
and far surpasses in variety all the other gems, with the exception
perhaps of quartz. To the yellow topaz it is decidedly superior in its
range of shades, and in some of its chrome-like tints it is without
an equal among the gems. This hue of chrome mixed with a faint tinge
of green is a delicate, yet gorgeous, shade, and is not often seen.
Stones of a canary-yellow are quite common, and perfect resemblances to
the Brazilian topaz are not rare. From these hues they pass insensibly
into brown and black. The transparent light-brown stones are often
modified in hue when exposed to the action of heat, and some of them
exhibit remarkable changes of color. M. Halpen, in 1866, exhibited to
the French Academy of Sciences a singular diamond of this description.
It was a stone of sixty grains weight, and of a whitish hue tinged
slightly with brown. But when it was exposed to the action of heat it
changed its tint to a fine rose-color, and retained it for six or eight
days, when it gradually returned to its natural hue. This remarkable
effect was not an accidental result, but was tried five times at the
Academy with success and without injury to the stone. In other colored
diamonds the action of fire often produces permanent effects, and
sometimes a brownish hue is converted into a decided red color. Buckman
saw a diamond with a large brown spot in its interior change to a
beautiful red, like that of the Balais ruby, after the stone had been
placed in borax and exposed to a red heat. Another stone, however, of
similar appearance, likewise exposed, changed to a permanent black,
to the great injury of the gem and dismay of the experimenter. The
red varieties of this mineral are rarely of deep tints, but when they
exhibit a decided red color they form the most gorgeous of gems. The
largest and finest of this description known is the ten-karat stone
purchased by the Emperor Paul of Russia for one hundred thousand
roubles. This gem may be considered the marvel of the mineral kingdom.
The princely collection of the late Mr. Hope possessed one of a
blood-red garnet shade, also a fine twelve and a half karat stone of an
apricot hue, besides several others of a beautiful hyacinth red, or of
a lilac pink.
The celebrated cabinet of gems belonging to the late Marquis de Drée
contained a large and beautiful rose-colored diamond. Prince Riccia,
of Naples, acquired in 1830 a very fine rose-colored brilliant of
fifteen karats weight. M. Halpen, in 1838, exhibited a magnificent
gem of this description of twenty-two and a half karats. Among the
crown jewels of France there are several splendid brilliants of a
peach-blossom hue, and there are also quite a number to be seen among
the princely caskets of Europe. It is, however, somewhat remarkable
that this gem, although possessing several shades of red, never, or
very rarely, occurs of a decided violet or purple color. Diamonds of a
light aqua-marine of greenish and bluish tints are not rare, but those
of a positive grass-green color are uncommon. Perfect stones of decided
green form the most magnificent gems of this color. The velvety green
flashed forth by the extraordinary power of the stone surpasses beyond
comparison the finest emeralds with their duller reflections. In fact,
we may term the splendid green diamond of forty karats, now in the
Green Vaults at Dresden, as being one of the five paragons among all
the gems of the world.
In the Museum of Natural History in Paris there are some small diamonds
of very fine shades of green, which were collected by the celebrated
Werner. Some of the diamonds which have a slight milky hue, when cut
so as to allow the play of light within the stone, present a very
beautiful appearance. The varied flashes of colored rays, in contrast
with the duller hues of the stone, appear like the charming effects of
the finest specimens of Siberian adularia, and are therefore entitled
to the name of aventurine diamonds.
The asterism, or star-like form of six rays, which is so beautifully
displayed by the sapphire when it assumes a certain form of internal
arrangement of crystallization, is sometimes, though very rarely,
witnessed in the diamond. There is one of this description to be seen
in the Museum of the _Jardin des Plantes_ in Paris.
The diamond is rarely found of a perfect shade of blue; but there are
now in Europe several magnificent gems of this description. Foremost of
all of them stood the famous blue diamond of 67²⁄₁₆ karats, belonging
to the French crown. This marvel of Nature’s work, with two other
diamonds of paler hue and lesser weight,--thirty-one and ten karats
respectively,--disappeared on that fatal night of September, 1792,
and have never since been discovered. At the present day, the finest
known is that which belongs to the princely collection of the late Mr.
Hope, and weighs 44¹⁄₄ karats. It is of a fine blue; but exhibits that
steel-like tint which is so often seen in sapphires. The next in value
and beauty is that which is preserved at Munich. It is a magnificent
gem of thirty-six karats weight, and of superb color.
The crystallized black diamond is a very rare stone; and, when
polished, it forms a unique gem, since it exhibits a remarkable
brilliancy, proceeding, as it were, from darkness itself. We do not now
refer to the compact variety, known as carbon or carbonado, which is
never found except in the amorphous form, but the crystalline variety,
which is of greater density and more homogeneous. The famous collector,
Dogni, possessed a very fine specimen of this kind which had been cut
with small facets, and exhibited a vivid eclat. It afterwards came into
the possession of Mr. Bapst, who disposed of it to Louis XVIII. for
the sum of twenty-four thousand francs. A large and unique diamond,
almost black, formerly belonged to the late Duke of York. Several of
the European mineralogical cabinets have interesting and valuable
collections of colored diamonds; but the finest is to be seen in the
Imperial Cabinet of Minerals at Vienna. This beautiful and complete
series, which illustrates the great range of the color-suite of the
gem, was the life-long labor of a Tyrolese gentleman, by the name of
Helmreicher. This enthusiastic amateur went to Brazil, and passed most
of his life in the mines, searching for the gems.
We will not fatigue our readers with long quotations of authors and
philosophers concerning the spiritual properties of this gem; but we
will briefly say, that a well-selected compilation of all these views
and speculations, extending back to very early times, would form a
chapter by itself, and quite as interesting as absurd. Even the good
sense of the Latin philosopher Pliny was affected so far as to indulge
in the belief that the gem was not only an antidote to poison, but also
freed the mind from vain fears. Late in mediæval times, the adamas
was invested with supernatural powers, and regarded as a spiritual
creation. And even in the commencement of the seventeenth century
Boetius de Boot, in his treatise on gems, asserts that the diamond
possesses wonderful metaphysical properties; but remarks that they do
not reside in the stone _per se_, but belong to the angelic spirits
whom it has pleased the Almighty to connect, in a mysterious manner,
with certain substances in nature.