CRYSTALLINE FORM
With the single exception of opal, the whole of the principal mineral
species used in jewellery are distinguished from glass and similar
substances by one fundamental difference: they are crystallized matter,
and the atoms composing them are regularly arranged throughout the
structure.
The words crystal and glass are employed in science in senses differing
considerably from those in popular use. The former of them is derived
from the Greek word κρύος, meaning ice, and was at one time used in
that sense. For instance, the old fourteenth-century reading of Psalm
cxlvii. 17, which appears in the authorized version as “He giveth his
ice like morsels,” ran “He sendis his kristall as morcels.” It was
also applied to the beautiful, lustrous quartz found among the eternal
snows of the Alps, since, on account of their limpidity, these stones
were supposed, as Pliny tells us, to consist of water congealed by the
extreme cold of those regions; such at the present day is the ordinary
meaning of the word. But, when early investigators discovered that a
salt solution on evaporation left behind groups of slender glistening
prisms, each very similar to the rest, they naturally—though, as we
now know, wrongly—regarded them as representing yet another form
of congealed water, and applied the same word to such substances.
Subsequent research has shown that these salts, as well as mineral
substances occurring with natural faces in nature, have in common the
fundamental property of regularity of arrangement of the constituent
atoms, and science therefore defines by the word crystal a substance in
which the structure is uniform throughout, and all the similar atoms
composing it are arranged with regard to the structure in a similar way.
The other word is yet more familiar; it denotes the transparent,
lustrous, hard, and brittle substance produced by the fusion of sand
with soda or potash or both which fills our windows and serves a
variety of useful purposes. Research has shown that glass, though
apparently so uniform in character, has in reality no regularity of
molecular arrangement. It is, in fact, a kind of mosaic of atoms,
huddled together anyhow, but so irregular is its irregularity that
it simulates perfect regularity. Science uses the word glass in this
widened meaning. Two substances may, as a matter of fact, have the
same chemical composition, and one be a crystal and the other a glass.
For example, quartz, if heated to a high temperature, may be fused and
converted into a glass. The difference in the two types of structure
may be illustrated by a comparison between a regiment of soldiers
drawn up on parade and an ordinary crowd of people.
The crystalline form is a visible sign of the molecular arrangement,
and is intimately associated with the directional physical properties,
such as the optical characters, cleavage, etc. A study of it is not
only of interest in itself, but also of great importance to the
lapidary who wishes to cut a stone to the best advantage, and it is,
moreover, of service in distinguishing stones when in the rough state.
[Illustration: FIG. 1.—Cubo-Octahedra.]
The development of natural faces on a crystal is far from being
haphazard, but is governed by the condition that the angles between
similar faces, whether on the same crystal or on different crystals,
are equal, however varying may be the shapes and the relative sizes
of the faces (Fig. 1), and by the tendency of the faces bounding
the crystal to fall into series with parallel edges, such series
being termed zones. Each species has a characteristic type of
crystallization, which may be referred to one of the following six
systems:—
1. _Cubic._—Crystals in this system can be referred to three edges,
which are mutually at right angles, and in which the directional
characters are identical in value. These principal edges are known
as axes. Some typical forms are the cube (Fig. 2), characteristic
of fluor; the octahedron (Fig. 3), characteristic of diamond and
spinel; the dodecahedron (Fig. 4), characteristic of garnet; and the
triakisoctahedron, or three-faced octahedron (Fig. 5).
[Illustration: FIG. 2.—Cube.]
[Illustration: FIG. 3.—Octahedron.]
[Illustration: FIG. 4.—Dodecahedron.]
All crystals belonging to this system are singly refractive.
2. _Tetragonal._—Such crystals can be referred to three axes, which are
mutually at right angles, but in only two of them are the directional
characters identical. A typical form is a four-sided prism, _mm_, of
square section, terminated by four triangular faces, _p_ (Fig. 6), the
usual shape of crystals of zircon and idocrase.
[Illustration: FIG. 5.—Triakisoctahedron, or Three-faced Octahedron.]
[Illustration: FIG. 6.—Tetragonal Crystal.]
Crystals belonging to this system are doubly refractive and uniaxial,
_i.e._ they have one direction of single refraction (cf. p. 45), which
is parallel to the unequal edge of the three mentioned above.
[Illustration: FIG. 7.—Two alternative sets of Axes in the Hexagonal
System.]
3. _Hexagonal._—Such crystals can be referred alternatively either
to a set of three axes, _X_, _Y_, _Z_ (Fig. 7), which lie in a plane
perpendicular to a fourth, _H_, and are mutually inclined at angles of
60°, or to a set of three, _a_, _b_, _c_, which are not at right angles
as in the cubic system, but in which the directional characters are
identical. The fourth axis in the first arrangement is equally inclined
to each in the second set of axes. Many important species crystallize
in this system—corundum (sapphire, ruby), beryl (emerald, aquamarine),
tourmaline, quartz, and phenakite. The crystals usually display a
six-sided prism, terminated by a single face, _c_, perpendicular to the
edge of the prism _m_ (Fig. 8), _e.g._ emerald, or by six or twelve
inclined faces, _p_ (Fig. 9), _e.g._ quartz, crystals of which are so
constant in form as to be the most familiar in the Mineral Kingdom.
Tourmaline crystals (Fig. 10) are peculiar because of the fact that
often one end is obviously to the eye flatter than the other.
[Illustration: FIGS. 8-10.—Hexagonal Crystals.]
Crystals belonging to this system are also doubly refractive and
uniaxial, the direction of single refraction being parallel to the
fourth axis mentioned above, and therefore also parallel to the prism
edge. Hence deeply coloured tourmaline, which strongly absorbs the
ordinary ray, must be cut with the table-facet parallel to the edge of
the prism.
[Illustration: FIG. 11.—Relation of the two directions of single
Refraction to the Axes in an Orthorhombic Crystal.]
4. _Orthorhombic._—Such crystals can be referred to three axes, which
are mutually at right angles, but in which each of the directional
characters are different. The crystals are usually prismatic in shape,
one of the axes being parallel to the prism edge. Topaz, peridot, and
chrysoberyl are the most important species crystallizing in this system.
Crystals belonging to this system are doubly refractive and biaxial,
_i.e._ they have two directions of single refraction (cf. p. 45). The
three axes _a_, _b_, _c_ (Fig. 11) are parallel respectively to the two
bisectrices of the directions of single refraction, and the direction
perpendicular to the plane containing those directions.
5. _Monoclinic._—Such crystals can be referred to three axes, one
of which is at right angles to the other two, which are, however,
themselves not at right angles. Spodumene (kunzite) and some moonstone
crystallize in this system.
Crystals belonging to this system are doubly refractive and biaxial,
but in this case the first axis alone is parallel to one of the
principal optical directions.
6. _Triclinic._—Such crystals have no edges at right angles, and the
optical characters are not immediately related to the crystalline form.
Some moonstone crystallizes in this system.
[Illustration: FIG. 12.—Twinned Octahedron.]
Crystals are often not single separate individuals. For instance,
diamond and spinel are found in flat triangular crystals with their
girdles cleft at the corners (Fig. 12). Each of such crystals is
really composed of portions of two similar octahedra, which are placed
together in such a way that each is a reflection of the other. Such
composite crystals are called twins or macles. Sometimes the twinning
is repeated, and the individuals may be so minute as to call for a
microscope for their perception.
A composite structure may also result from the conjunction of
numberless minute individuals without any definite orientation, as in
the case of chalcedony and agate. So by supposing the individuals to
become infinitesimally small, we pass to a glass-like substance.
It is often a peculiarity of crystals of a species to display a typical
combination of natural faces. Such a combination is known as the habit
of the species, and is often of service for the purpose of identifying
stones before they are cut. Thus, a habit of diamond and spinel is
an octahedron, often twinned, of garnet a dodecahedron, of emerald a
flat-ended hexagonal prism, and so on.
It is one of the most interesting and remarkable features connected
with crystallization that the composition and the physical
characters—for instance, the refractive indices and specific
gravity—may, without any serious disturbance of the molecular
arrangement, vary considerably owing to the more or less complete
replacement of one element by another closely allied to it. That is
the cause of the range of the physical characters which has been
observed in such species as tourmaline, peridot, spinel, etc. The
principal replacements in the case of the gem-stones are ferric oxide,
Fe_{2}O_{3}, by alumina, Al_{2}O_{3}, and ferrous oxide, FeO, by
magnesia, MgO.
A list of the principal gem-stones, arranged by their chemical
composition, is given in Table I at the end of the book.