ABSORPTION EFFECTS: COLOUR, DICHROISM, ETC.
When white light passes through a cut stone, colour effects result
which arise from a variety of causes. The most obvious is the
fundamental colour of the stone, which is due to its selective
absorption of the light passing through it, and would characterize
it before it was cut. Intermingled with the colour in a transparent
stone is the dispersive effect known as ‘fire,’ which has already
been discussed (p. 20). In many instances the want of homogeneity is
responsible for some peculiar effects such as opalescence, chatoyancy,
and asterism. These phenomena will now be considered in fuller detail.
COLOUR
All substances absorb light to some extent. If the action is slight and
affects equally the whole of the visible spectrum, the stone appears
white or colourless. Usually some portion is more strongly absorbed
than the rest, and the stone seems to be coloured. What is the precise
tint depends not only upon the portions transmitted through the stone,
but also upon their relative intensities. The eye, unlike the ear, has
not the power of analysis and it cannot of itself determine how a
composite colour has been made up. Indeed, so far as it is concerned,
any colour may be exactly matched by compounding in certain proportions
three simple primary colours—red, yellow, and violet. Alexandrite, a
variety of chrysoberyl, is a curious and instructive case. The balance
in the spectrum of light transmitted through it is such that, whereas
in daylight such stones appear green, in artificial light, especially
in gas-light, they are a pronounced raspberry-red (Plate XXVII,
Figs. 11, 13). The phenomenon is intensified by the strong dichroism
characteristic of this species.
The colour is the least reliable character that may be employed for the
identification of a stone, since it varies considerably in the same
species, and often results from the admixture of some metallic oxide,
which has no essential part in the chemical composition and is present
in such minute quantities as to be almost imperceptible by analysis.
Who would, for instance, imagine from their appearance that stones so
markedly diverse in hue as ruby and sapphire were really varieties of
the same species, corundum? Again, quartz, in spite of the simplicity
of its composition, displays extreme differences of tint. Nevertheless,
certain varieties do possess a distinctive colour, emerald being
the most striking example, and in other cases the trained eye can
appreciate certain characteristic subtleties of shade. At any rate, the
colour is the most obvious of the physical characters, and serves to
provide a rough division of the species, and accordingly in Table II at
the end of the book the gem-stones are arranged by their usual tints.
DICHROISM
The two rays into which a doubly refractive stone splits up a ray of
light are often differently absorbed by it, and in consequence appear
on emergence differently coloured; such stones are said to be dichroic.
The most striking instance is a deep-brown tourmaline, which, except
in very thin sections, is quite opaque to the ordinary ray. The light
transmitted by a plate cut parallel to the crystallographic axis is
therefore plane-polarized; before the invention by Nicol of the prism
of Iceland-spar known by his name this was the ordinary method of
obtaining light of this character (cf. p. 43). Again, in the case of
kunzite and cordierite the difference in colour is so marked as to be
obvious to the unaided eye; but where the contrast is less pronounced
we require the use of an instrument called a dichroscope, which enables
the twin colours to be seen side by side.
[Illustration: FIG. 29.—Dichroscope (actual size).]
[Illustration: FIG. 30.—Field of the Dichroscope.]
Fig. 29 illustrates in section the construction of a dichroscope. The
instrument consists essentially of a rhomb of Iceland-spar, _S_, of
such a length as to give two contiguous images (Fig. 30) of a square
hole, _H_, in one end of the tube containing it. In some instruments
the terminal faces of the rhomb are ground at right angles to its
length, but usually, as in that depicted, prisms of glass, _G_, are
cemented on to the two ends. A cap _C_, with a slightly larger hole,
which is circular in shape, fits on the end of the tube, and can be
moved up and down it and revolved round it, as desired. The stone, _R_,
to be tested may be directly attached to it by means of some kind of
wax or cement in such a way that light which has traversed it passes
into the window, _H_, of the instrument; the cap at the same time
permits of the rotation of the stone about the axis of the main tube of
the instrument. The dichroscope shown in the figure has a still more
convenient arrangement: it is provided with an additional attachment,
_A_, by means of which the stone can be turned about an axis at right
angles to the length of the tube, and thus examined in different
directions. At the other end of the main tube is placed a lens, _L_, of
low power for viewing the twin images: the short tube containing it can
be pushed in and out for focusing purposes. Many makers now place the
rhomb close to the lens, _L_, and thereby require a much smaller piece
of spar; material suitable for optical purposes is fast growing scarce.
Suppose that a plate of tourmaline cut parallel to its crystallographic
axis is fastened to the cap and the latter rotated. We should notice,
on looking through the instrument, that in the course of a complete
revolution there are two positions, orientated at right angles to
one another, in which the tints of the two images are identical, the
positions of greatest contrast of tint being midway between. If we
examine a uniaxial stone in a direction at right angles to its optic
axis we obtain the colours corresponding to the ordinary and the
extraordinary rays. In any direction less inclined to the axis we
still have the colour for the ordinary ray, but the other colour is
intermediate in tint between it and that for the extraordinary ray.
The phenomenon presented by a biaxial stone is more complex. There
are three principal colours which are visible in differing pairs in
the three principal optical directions; in other directions the tints
seen are intermediate between the principal colours. Since biaxial
stones have three principal colours, they are sometimes said to be
trichroic or pleochroic, but in any single direction they have two
twin colours and show dichroism. No difference at all will be shown in
directions in which a stone is singly refractive, and it is therefore
always advisable to examine a stone in more than one direction lest
the first happens to be one of single refraction. For determinative
purposes it is not necessary to note the exact shades of tint of the
twin colours, because they vary with the inherent colour of the stone,
and are therefore not constant for the same species; we need only
observe, when the stone is tested with the dichroscope, whether there
is any variation of colour, and, if so, its strength. Dichroism is a
result of double refraction, and cannot exist in a singly refractive
stone. The converse, however, is not true and it by no means follows
that, because no dichroism can be detected in a stone, it is singly
refractive. A colourless stone, for instance, cannot possibly be
dichroic, and many coloured, doubly refractive stones—for example,
zircon—exhibit no dichroism, or so little that it is imperceptible.
The character is always the better displayed, the deeper the inherent
colour of the stone. The deep-green alexandrite, for instance, is far
more dichroic than the lighter coloured varieties of chrysoberyl.
If the stone is attached to the cap of the instrument, the table should
be turned towards it so as to assure that the light passing into the
instrument has actually traversed the stone. If little light enters
through the opposite coign, a drop of oil placed thereon will overcome
the difficulty (cf. p. 46). It is also necessary, for reasons mentioned
above, to examine the stone in directions as far as possible across
the girdle also. A convenient, though not strictly accurate, method
is to lay the stone with the table facet on a table and examine the
light which has entered the stone and been reflected at that facet. The
stone may easily be rotated on the table, and observations thus made
in different directions in the stone. Care must be exercised in the
case of a faceted stone not to mistake the alteration in colour due to
dispersion for a dichroic effect, and the stone must be placed close to
the instrument during an observation, because otherwise the twin rays
traversing the instrument may have taken sensibly different directions
in the stone.
Dichroism is an effective test in the case of ruby; its twin
colours—purplish and yellowish red—are in marked contrast, and readily
distinguish it from other red stones. Again, one of the twin colours
of sapphire is distinctly more yellowish than the other; the blue
spinel, of which a good many have been manufactured during recent
years, is singly refractive, and, of course, shows no difference of
tint in the dichroscope.
Table VI at the end of the book gives the strength of the dichroism of
the gem-stones.
ABSORPTION SPECTRA
A study of the chromatic character of the light transmitted by a
coloured stone is of no little interest. As was stated above, the eye
has not the power of analysing light, and to resolve the transmitted
rays into their component parts an instrument known as a spectroscope
is needed. The small ‘direct-vision’ type has ample dispersion for this
purpose. It is advantageous to employ by preference the diffraction
rather than the prism form, because in the former the intervals in the
resulting spectrum corresponding to equal differences of wave-length
are the same, whereas in the latter they diminish as the wave-length
increases and accordingly the red end of the spectrum is relatively
cramped.
The absorptive properties of all doubly refractive coloured substances
vary more or less with the direction in which light traverses them
according to the amount of dichroism that they possess, but the
variation is not very noticeable unless the stone is highly dichroic.
If the light transmitted by a deep-coloured ruby be examined with a
spectroscope it will be found that the whole of the green portion of
the spectrum is obliterated (Fig. 31), while in the case of a sapphire
only a small portion of the red end of the spectrum is absorbed.
Alexandrite affords especial interest. In the spectrum of the light
transmitted by it, the violet and the yellow are more or less strongly
absorbed, depending upon the direction in which the rays have passed
through the stone (Fig. 31), and the transmitted light is mainly
composed of two portions—red and green. The apparent colour of the
stone depends, therefore, upon which of the two predominates. In
daylight the resultant colour is green flecked with red and orange,
the three principal absorptive tints (cf. p. 235), but in artificial
light, which is relatively stronger in the red portion of the spectrum,
the resultant colour is a raspberry-red, and there is less apparent
difference in the absorptive tints (cf. Plate XXVII, Figs. 11, 13).
[Illustration: FIG. 31.—Absorption Spectra.]
In all the spectra just considered, and in all like them, the portions
that are absorbed are wide, the passage from blackness to colour is
gradual, and the edges deliminating them are blurred. In the spectra of
certain zircons and in almandine garnet the absorbed portions, or bands
as they are called, are narrow, and, moreover, the transition from
blackness to colour is sharp and abrupt; such stones are therefore said
to display absorption-bands. Church in 1866 was the first to notice
the bands shown by zircon (Fig. 31). Sorby thought they portended the
existence of a new element, to which he gave the name jargonium, but
subsequently discovered that they were caused by the presence of a
minute trace of uranium. A yellowish-green zircon shows the phenomenon
best, and it has all the bands shown in the figure. The spectrum varies
slightly but almost imperceptibly with the direction in the stone.
Others show the bands in the yellow and green, while others show only
those in the red, and some only one of them. The bands are not confined
to stones of any particular colour, or amount of double refraction.
Again, many zircons show no bands at all, so that their absence by no
means precludes the stone from being a zircon.
Almandine is characterized by a different spectrum (Fig. 31). The band
in the yellow is the most conspicuous, and is no doubt responsible for
the purple hue of a typical almandine. The spectrum varies in strength
in different stones. Rhodolite (p. 214), a garnet lying between
almandine and pyrope, displays the same bands, and indications of them
may be detected in the spectra of pyropes of high refraction.
[Illustration: _PLATE IV_
JEWELLERY DESIGNS]