PHYSICAL PROPERTIES.
A--CRYSTALLINE STRUCTURE.
Before proceeding to the study of precious stones as individual gems,
certain physical properties common to all must be discussed, in order to
bring the gems into separate classes, not only because of some chemical
uniformity, but also because of the unity which exists between their
physical formation and properties.
The first consideration, therefore, may advisedly be that of their
crystals, since their crystalline structure forms a ready means for the
classification of stones, and indeed for that of a multitudinous variety
of substances.
It is one of the many marvellous phenomena of nature that mineral, as
well as many vegetable and animal substances, on entering into a state
of solidity, take upon themselves a definite form called a crystal.
These crystals build themselves round an axis or axes with wonderful
regularity, and it has been found, speaking broadly, that the same
substance gives the same crystal, no matter how its character may be
altered by colour or other means. Even when mixed with other
crystallisable substances, the resulting crystals may partake of the two
varieties and become a sort of composite, yet to the physicist they are
read like an open book, and when separated by analysis they at once
revert to their original form. On this property the analyst depends
largely for his results, for in such matters as food adulteration, etc.,
the microscope unerringly reveals impurities by means of the crystals
alone, apart from other evidences.
It is most curious, too, to note that no matter how large a crystal may
be, when reduced even to small size it will be found that the crystals
are still of the same shape. If this process is taken still further, and
the substance is ground to the finest impalpable powder, as fine as
floating dust, when placed under the microscope each speck, though
perhaps invisible to the naked eye, will be seen a perfect crystal, of
the identical shape as that from which it came, one so large maybe that
its planes and angles might have been measured and defined by rule and
compass. This shows how impossible it is to alter the shape of a
crystal. We may dissolve it, pour the solution into any shaped vessel or
mould we desire, recrystallise it and obtain a solid sphere, triangle,
square, or any other form; it is also possible, in many cases, to
squeeze the crystal by pressure into a tablet, or any form we choose,
but in each case we have merely altered the _arrangement_ of the
crystals, so as to produce a differently shaped _mass_, the crystals
themselves remaining individually as before. Such can be said to be one
of the laws of crystals, and as it is found that every substance has its
own form of crystal, a science, or branch of mineralogy, has arisen,
called "crystallography," and out of the conglomeration of confused
forms there have been evolved certain rules of comparison by which all
known crystals may be classed in certain groups.
This is not so laborious a matter as would appear, for if we take a
substance which crystallises in a cube we find it is possible to draw
nine symmetrical planes, these being called "planes of symmetry," the
intersections of one or more of which planes being called "axes of
symmetry." So that in the nine planes of symmetry of the cube we get
three axes, each running through to the opposite side of the cube. One
will be through the centre of a face to the opposite face; a second will
be through the centre of one edge diagonally; the third will be found in
a line running diagonally from one point to its opposite. On turning the
cube on these three axes--as, for example, a long needle running through
a cube of soap--we shall find that four of the six identical faces of
the cube are exposed to view during each revolution of the cube on the
needle or axis.
These faces are not necessarily, or always, planes, or flat, strictly
speaking, but are often more or less curved, according to the shape of
the crystal, taking certain characteristic forms, such as the square,
various forms of triangles, the rectangle, etc., and though the crystals
may be a combination of several forms, all the faces of any particular
form are similar.
All the crystals at present known exhibit differences in their planes,
axes and lines of symmetry, and on careful comparison many of them are
found to have some features in common; so that when they are sorted out
it is seen that they are capable of being classified into thirty-three
groups. Many of these groups are analogous, so that on analysing them
still further we find that all the known crystals may be classed in six
separate systems according to their planes of symmetry, and all stones
of the same class, no matter what their variety or complexity may be,
show forms of the same group. Beginning with the highest, we have--(1)
the cubic system, with nine planes of symmetry; (2) the hexagonal, with
seven planes; (3) the tetragonal, with five planes; (4) the rhombic,
with three planes; (5) the monoclinic, with one plane; (6) the
triclinic, with no plane of symmetry at all.
In the first, the cubic--called also the isometric, monometric, or
regular--there are, as we have seen, three axes, all at right angles,
all of them being equal.
The second, the hexagonal system--called also the rhombohedral--is
different from the others in having four axes, three of them equal and
in one plane and all at 120 deg. to each other; the fourth axis is not
always equal to these three. It may be, and often is, longer or shorter.
It passes through the intersecting point of the three others, and is
perpendicular or at right angles to them.
The third of the six systems enumerated above, the tetragonal--or the
quadratic, square prismatic, dimetric, or pyramidal--system has three
axes like the cubic, but, in this case, though they are all at right
angles, two only of them are equal, the third, consequently, unequal.
The vertical or principal axis is often much longer or shorter in this
group, but the other two are always equal and lie in the horizontal
plane, at right angles to each other, and at right angles to the
vertical axis.
The fourth system, the rhombic--or orthorhombic, or prismatic, or
trimetric--has, like the tetragonal, three axes; but in this case, none
of them are equal, though the two lateral axes are at right angles to
each other, and to the vertical axis, which may vary in length, more so
even than the other two.
The fifth, the monoclinic--or clinorhombic, monosymmetric, or
oblique--system, has also three axes, all of them unequal. The two
lateral axes are at right angles to each other, but the principal or
vertical axis, which passes through the point of intersection of the two
lateral axes, is only at right angles to one of them.
In the sixth and last system, the triclinic--or anorthic, or
asymmetric--the axes are again three, but in this case, none of them are
equal and none at right angles.
It is difficult to explain these various systems without drawings, and
the foregoing may seem unnecessarily technical. It is, however,
essential that these particulars should be clearly stated in order
thoroughly to understand how stones, especially uncut stones, are
classified. These various groups must also be referred to when dealing
with the action of light and other matters, for in one or other of them
most stones are placed, notwithstanding great differences in hue and
character; thus all stones exhibiting the same crystalline structure as
the diamond are placed in the same group. Further, when the methods of
testing come to be dealt with, it will be seen that these particulars of
grouping form a certain means of testing stones and of distinguishing
spurious from real. For if a stone is offered as a real gem (the true
stone being known to lie in the highest or cubic system), it follows
that should examination prove the stone to be in the sixth system, then,
no matter how coloured or cut, no matter how perfect the imitation, the
test of its crystalline structure stamps it readily as false beyond all
shadow of doubt--for as we have seen, no human means have as yet been
forthcoming by which the crystals can be changed in form, only in
arrangement, for a diamond crystal _is_ a diamond crystal, be it in a
large mass, like the brightest and largest gem so far discovered--the
great Cullinan diamond--or the tiniest grain of microscopic
diamond-dust, and so on with all precious stones. So that in future
references, to avoid repetition, these groups will be referred to as
group 1, 2, and so on, as detailed here.