PHYSICAL PROPERTIES.
C--LIGHT.
Probably the most important of the many important physical properties
possessed by precious stones are those of light and its effects, for to
these all known gems owe their beauty, if not actual fascination.
When light strikes a cut or polished stone, one or more of the following
effects are observed:--it may be transmitted through the stone,
diaphaneity, as it is called; it may produce single or double
refraction, or polarisation; if reflected, it may produce lustre or
colour; or it may produce phosphorescence; so that light may be (1)
transmitted; (2) reflected; or produce (3) phosphorescence.
(1) TRANSMISSION.--In transmitted light we have, as stated above, single
or double refraction, polarisation, and diaphaneity.
To the quality of _refraction_ is due one of the chief charms of certain
precious stones. It is not necessary to explain here what refraction is,
for everyone will be familiar with the refractive property of a
light-beam when passing through a medium denser than atmospheric air. It
will be quite sufficient to say that all the rays are not equal in
refractive power in all substances, so that the middle of the spectrum
is generally selected as the mean for indexing purposes.
It will be seen that the stones in the 1st, or cubic system, show single
refraction, whereas those of all other systems show double refraction;
thus, light, in passing through their substance, is deviated, part of it
going one way, the other portion going in another direction--that is, at
a slightly different angle--so that this property alone will isolate
readily all gems belonging to the 1st system.
A well-known simple experiment in physics shows this clearly. A mark on
a card or paper is viewed through a piece of double-refracting spar
(Iceland spar or clear calcite), when the mark is doubled and two
appear. On rotating this rhomb of spar, one of these marks is seen to
revolve round the other, which remains stationary, the moving mark
passing further from the centre in places. When the spar is cut and used
in a certain direction, we see but one mark, and such a position is
called its optical axis.
_Polarisation_ is when certain crystals possessing double refraction
have the power of changing light, giving it the appearance of poles
which have different properties, and the polariscope is an instrument in
which are placed pieces of double-refracting (Iceland) spar, so that all
light passing through will be polarised.
Since only crystals possessing the property of double refraction show
polarisation, it follows that those of the 1st, or cubic system--in
which the diamond stands a prominent example--fail to become polarised,
so that when such a stone is placed in the polariscope and rotated, it
fails _at every point_ to transmit light, which a double-refracting gem
allows to pass except when its optical axis is placed in the axis of the
polariscope, but this will be dealt with more fully when the methods of
testing the stones come to be considered.
_Diaphaneity_, or the power of transmitting light:--some rather fine
trade distinctions are drawn between the stones in this class, technical
distinctions made specially for purposes of classification, thus:--a
"non-diaphanous" stone is one which is quite opaque, no light of any
kind passing through its substance; a "diaphanous" stone is one which is
altogether transparent; "semi-diaphanous" means one not altogether
transparent, and sometimes called "sub-transparent." A "translucent"
stone is one in which, though light passes through its substance, sight
is not possible through it; whilst in a "sub-translucent" stone, light
passes through it, but only in a small degree.
The second physical property of light is seen in those stones which owe
their beauty or value to REFLECTION: this again may be dependent on
Lustre, or Colour.
~Lustre.~--This is an important characteristic due to reflection, and of
which there are six varieties:--([alpha]) adamantine (which some
authorities, experts and merchants subdivide as detailed below);
([beta]) pearly; ([gamma]) silky; ([delta]) resinous; ([epsilon])
vitreous; ([zeta]) metallic. These may be described:--
([alpha]) Adamantine, or the peculiar lustre of the diamond, so called
from the lustre of adamantine spar, which is a form of corundum (as is
emery) with a diamond-like lustre, the hard powder of which is used in
polishing diamonds. It is almost pure anhydrous alumina (Al_{2}O_{3})
and is, roughly, four times as heavy as water. The lustre of this is the
true "adamantine," or diamond, brilliancy, and the other and impure
divisions of this particular lustre are: _splendent_, when objects are
reflected perfectly, but of a lower scale of perfection than the true
"adamantine" standard, which is absolutely flawless. When still lower,
and the reflection, though maybe fairly good, is somewhat "fuzzy," or is
confused or out of focus, it is then merely _shining_; when still less
distinct, and no trace of actual reflection is possible (by which is
meant that no object can be reproduced in any way to define it, as it
could be defined in the reflection from still water or the surface of a
mirror, even though imperfectly) the stone is then said to _glint_ or
_glisten_. When too low in the scale even to glisten, merely showing a
feeble lustre now and again as the light is reflected from its surface
in points which vary with the angle of light, the stone is then said to
be _glimmering_. Below this, the definitions of lustre do not go, as
such stones are said to be _lustreless_.
([beta]) Pearly, as its name implies, is the lustre of a pearl.
([gamma]) Silky, possessing the sheen of silk, hence its name.
([delta]) Resinous, also explanatory in its name; amber and the like
come in this variety.
([epsilon]) Vitreous. This also explains itself, being of the lustre of
glass, quartz, etc.; some experts subdividing this for greater defining
accuracy into the "sub-vitreous" or lower type, for all but perfect
specimens.
([zeta]) Metallic or Sub-metallic. The former when the lustre is perfect
as in gold; the latter when the stones possess the less true lustre of
copper.
~Colour.~--Colour is an effect entirely dependent upon light, for in the
total absence of light, such as in black darkness, objects are
altogether invisible to the normal human eye. In daylight, also, certain
objects reflect so few vibrations of light, or none, that they appear
grey, black, or jet-black; whilst those which reflect all the rays of
which light is composed, and in the same number of vibrations, appear
white. Between these two extremes of _none_ and _all_ we find a
wonderful play and variety of colour, as some gems allow the red rays
only to pass and therefore appear red; others allow the blue rays only
and these appear blue, and so on, through all the shades, combinations
and varieties of the colours of which light is composed, as revealed by
the prism. But this is so important a matter that it demands a chapter
to itself.
The third physical property of light, PHOSPHORESCENCE, is the property
possessed by certain gems and minerals of becoming phosphorescent on
being rubbed, or on having their temperature raised by this or other
means.
It is difficult to say exactly whether this is due to the heat, the
friction, or to electricity. Perhaps two or all of these may be the
cause, for electricity is developed in some gems--such as the topaz--by
heat, and heat by electricity, and phosphorescence developed by both.
For example, if we rub together some pulverised fluorspar in the dark,
or raise its temperature by the direct application of heat, such as from
a hot or warm iron, or a heated wire, we at once obtain excellent
phosphorescence. Common quartz, rubbed against a second piece of the
same quartz in the dark, becomes highly phosphorescent. Certain gems,
also, when merely exposed to light--sunlight for preference--then taken
into a darkened room, will glow for a short time. The diamond is one of
the best examples of this kind of phosphorescence, for if exposed to
sunlight for a while, then covered and rapidly taken into black
darkness, it will emit a curious phosphorescent glow for from one to ten
seconds; the purer the stone, the longer, clearer and brighter the
result.