lk, 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 int