PHYSICAL PROPERTIES.
D--COLOUR.
Colour is one of the most wonderful effects in nature. It is an
attribute of light and is not a part of the object which appears to be
coloured; though all objects, by their chemical or physical composition,
determine the number and variety of vibrations passed on or returned to
the eye, thus fixing their own individual colours.
We have also seen that if an _equal_ light-beam becomes obstructed in
its passage by some substance which is denser than atmospheric air, it
will become altered in its direction by refraction or reflection, and
polarised, each side or pole having different properties.
Polarised light cannot be made again to pass in a certain direction
through the crystal which has polarised it; nor can it again be
reflected at a particular angle; so that in double-refracting crystals,
these two poles, or polarised beams, are different in colour, some
stones being opaque to one beam but not to the other, whilst some are
opaque to both.
This curious phenomenon, with this brief, though somewhat technical
explanation, shows the cause of many of the great charms in precious
stones, for when viewed at one angle they appear of a definite colour,
whilst at another angle they are just as decided in their colour, which
is then entirely different; and as these angles change as the eye
glances on various facets, the stone assumes a marvellous wealth of the
most brilliant and intense colour of kaleidoscopic variety, even in a
stone which may itself be absolutely clear or colourless to ordinary
light.
Such an effect is called pleochroism, and crystals which show variations
in their colour when viewed from different angles, or by transmitted
light, are called pleochroic, or pleochromatic--from two Greek words
signifying "to colour more." To aid in the examination of this
wonderfully beautiful property possessed by precious stones, a little
instrument has been invented called the dichroscope, its name showing
its Greek derivation, and meaning--"to see colour twice" (twice, colour,
to see). It is often a part of a polariscope; frequently a part also of
the polarising attachment to the microscope, and is so simple and
ingenious as to deserve detailed explanation.
In a small, brass tube is fixed a double-image prism of calcite or
Iceland spar, which has been achromatised--that is, clear, devoid of
colour--and is therefore capable of transmitting light without showing
any prismatic effect, or allowing the least trace of any except the
clear light-beam to pass through. At one end of this tube there is a
tiny square hole, the opposite end carrying a small convex lens, of such
a strength or focus as to show the square hole in true focus, that is,
with perfectly sharp definition, even up to the corners of the square.
On looking through the tube, the square hole is duplicated, two squares
being seen. The colours of a gem are tested by the stone being put in
front of this square, when the two colours are seen quite distinctly.
Not only is this a simple means of judging colour, but it enables a
stone to be classified readily. For if the dichroscope shows two images
of _the same_ colour, then it may possibly be a carbuncle, or a diamond,
as the case may be--for single-refracting stones, of the first or cubic
system, show two images of _the same_ colour. But if these two colours
are different, then it must be a double-refracting stone, and according
to the particular colours seen, so is the stone classified, for each
stone has its own identical colour or colours when viewed through this
small but useful instrument.
How clear and distinct are these changes may be viewed without it in
substances strongly dichroic; for instance, if common mica is viewed in
one direction, it is transparent as polished plate-glass, whilst at
another angle, it is totally opaque. Chloride of palladium also is
blood-red when viewed parallel to its axis, and transversely, it is a
remarkably bright green. The beryl also, is sea-green one way and a
beautiful blue another; the yellow chrysoberyl is brown one way and
yellow with a greenish cast when viewed another way. The pink topaz
shows rose-colour in one direction and yellow in another. These are
perhaps the most striking examples, and are mostly self-evident to the
naked eye, whilst in other cases, the changes are so delicate that the
instrument must be used to give certainty; some again show changes of
colour as the stone is revolved in the dichroscope, or the instrument
revolved round the stone.
Some stones, such as the opal, split up the light-beams as does a
prism, and show a wonderful exhibition of prismatic colour, which is
technically known as a "play of colour." The descriptive term
"opalescence" is self-suggesting as to its origin, which is the "noble"
or "precious" opal; this radiates brilliant and rapidly changing
iridescent reflections of blue, green, yellow and red, all blending
with, and coming out of, a curious silky and milky whiteness, which is
altogether characteristic. The moonstone is another example of this
peculiar feature which is possessed in a more or less degree by all the
stones in the class of pellucid jewels, but no stone or gem can in any
way even rival the curious mixture of opaqueness, translucency,
silkiness, milkiness, fire, and the steadfast changeable and prismatic
brilliance of colour of the precious opal. The other six varieties of
opal are much inferior in their strange mixture of these anomalies of
light and colour. Given in order of value, we have as the second, the
"fire" opal with a red reflection, and, as a rule, that only. The third
in value is the "common" opal, with the colours of green, red, white and
yellow, but this is easily distinguishable from the "noble" or
"precious" variety in that the common opal does not possess that
wonderful "play" of colour. The fourth variety is called the
"semi-opal," which is really like the third variety, the "common," but
of a poorer quality and more opaque. The fifth variety in order of
value, is that known as the "hydrophane," which has an interesting
characteristic in becoming transparent when immersed in water, and only
then. The sixth is the "hyalite," which has but a glassy or vitreous
lustre, and is found almost exclusively in the form of globules, or
clusters of globules, somewhat after the form and size of bunches of
grapes; hence the name "botryoidal" is often applied to this variety.
The last and commonest of all the seven varieties of opal is somewhat
after the shape of a kidney (reniform), or other irregular shape,
occasionally almost transparent, but more often somewhat translucent,
and very often opaque. This seventh class is called "menilite," being
really an opaline form of quartz, originally found at Menilmontant,
hence its name (_Menil_, and Greek _lithos_, stone). It is a curious
blue on the exterior of the stone, brown inside.
History records many magnificent and valuable opals, not the least of
which was that of Nonius, who declined to give it to Mark Antony,
choosing exile rather than part with so rare a jewel, which Pliny
describes as being existent in his day, and of a value which, in present
English computation, would exceed one hundred thousand pounds.
Many other stones possess one or more properties of the opal, and are
therefore considered more or less opalescent. This "play of colour" and
"opalescence," must not be confused with "change of colour." The two
first appear mostly in spots and in brilliant points or flashes of
coloured light, or "fire" as it is termed. This fire is constantly on
the move, or "playing," whereas "change of colour," though not greatly
dissimilar, is when the fire merely travels over broader surfaces, each
colour remaining constant, such as when directly moving the stone, or
turning it, when the broad mass of coloured light slowly changes,
usually to its complementary. Thus in this class of stone, subject to
"change of colour," a green light is usually followed by its
complementary, red, yellow by purple, blue by orange, green by brown,
orange by grey, purple by broken green, with all the intermediary shades
of each.
Thus when the line of sight is altered, or the stone moved, never
otherwise, the colours chase one another over the surface of the gem,
and mostly in broad splashes; but in those gems possessing "play of
colour," strictly speaking, whilst the stone itself remains perfectly
still, and the sight is fixed unwaveringly upon it, the pulsations of
the blood in the eyes, with the natural movements of the eyes and
eyelids, even in a fixed, steady glance, are quite sufficient to create
in the stone a display of sparks and splashes of beautiful fiery light
and colour at every tremor.
The term "iridescence" is used when the display of colour is seen on the
surface, rather than coming out of the stone itself. The cause of this
is a natural, or in some cases an accidental, breaking of the surface of
the stone into numerous cobweb-like cracks; these are often of
microscopic fineness, only perceptible under moderately high powers.
Nevertheless they are quite sufficient to interfere with and refract the
light rays and to split them up prismatically. In some inferior stones
this same effect is caused or obtained by the application of a gentle
heat, immersion in chemicals, subjection to "X rays" and other strong
electric influence, and in many other ways. As a result, the stone is
very slightly expanded, and as the molecules separate, there appear on
the surface thousands, perhaps millions, of microscopic fissures running
at all angles, so that no matter from what position the stone may be
viewed, a great number of these fissures are certain to split up the
light into prismatic colours causing brilliant iridescence. Similar
fissures may often be seen with the naked eye on glass, especially if
scorched or cooled too rapidly (chilled), and on the surface of clear
spar and mica, their effects being of extreme interest, from a colour
point of view, at least.