LUSTRE AND SHEEN
It has been already stated that whenever light in one medium falls
upon the surface separating it from another medium some of the light
is reflected within the first, while the remainder passes out into
the second medium, except when the first is of lower refractivity
than the second and light falls at an angle greater than that of
total-reflection. Similarly, when light impinges upon a cut stone
some of it is reflected and the remainder passes into the stone. What
is the relative amount of reflected light depends upon the nature of
the stone—its refractivity and hardness—and determines its lustre;
the greater the amount the more lustrous will the stone appear. There
are different kinds of lustre, and the intensity of each depends on
the polish of the surface. From a dull, _i.e._ an uneven, surface the
reflected light is scattered, and there are no brilliant reflections.
All gem-stones take a good polish, and have therefore, so long as the
surface retains its polish, considerable brilliancy; turquoise, on
account of its softness, is always comparatively dull.
The different kinds of lustre are—
(1) Adamantine, characteristic of diamond.
(2) Vitreous, as seen on the surface of fractured glass.
(3) Resinous, as shown by resins.
Zircon and demantoid, the green garnet called by jewellers “olivine,”
alone among gem-stones have a lustre approaching that of diamond. The
remainder all have a vitreous lustre, though varying in degree, the
harder and the more refractive species being on the whole the more
lustrous.
Some stones—for instance, a cinnamon garnet—appear to have a certain
greasiness in the lustre, which is caused by stray reflections from
inclusions or other breaks in the homogeneity of the interior. A pearly
lustre, which arises from cleavage cracks and is typically displayed
by the cleavage face of topaz, would be seen in a cut stone only when
flawed.
Certain corundums when viewed in the direction of the
crystallographical axis display six narrow lines of light radiating at
angles of 60° from a centre in a manner suggestive of the conventional
representations of stars. Such stones are consequently known as
asterias, or more usually star-stones—star-rubies or star-sapphires, as
the case may be, and the phenomenon is called asterism. These stones
have not a homogeneous structure, but contain tube-like cavities
regularly arranged at angles of 60° in planes at right angles to the
crystallographical axis. The effect is best produced when the stones
are cut _en cabochon_ perpendicular to that axis.
Chatoyancy is a somewhat similar phenomenon, but in this case the
fibres or cavities are parallel to a single direction, and a single
broadish band is displayed at right angles to it. Cat’s-eyes, as these
stones are termed, are cut _en cabochon_ parallel to the fibres. The
true cat’s-eye (Plate XXIX, Fig. 1) is a variety of chrysoberyl, but
the term is also often applied to quartz showing a similar appearance.
The latter is really a fibrous mineral, such as asbestos, which has
become converted into silica. The beautiful tiger’s-eye from South
Africa is a silicified crocidolite, the original blue colour of which
has been altered by oxidation to golden brown. Recently tourmalines
have been discovered which are sufficiently fibrous in structure to
display an effective chatoyancy.
The milky sheen of moonstone (Plate XXIX, Fig. 4) owes its effect to
reflections from twin lamellæ. The wonderful iridescence which is
the glory of opal, and is therefore termed opalescence, arises from
a structure which is peculiar to that species. Opal is a solidified
jelly; on cooling it has become riddled with extremely thin cracks,
which were subsequently filled with similar material of slightly
different refractivity, and thus it consists of a series of films. At
the surface of each film interference of light takes place just as
at the surface of a soap-bubble, and the more evenly the films are
spaced apart the more uniform is the colour displayed, the actual tint
depending upon the thickness of the films traversed by the light giving
rise to the phenomenon.