light that falls upon the surface of a diamond at any low angle is
reflected, hence the keenness of its luster. If a diamond and some other
white stone, say a white sapphire, are held so as to reflect at the same
time images of an incandescent light into the eye of the observer, such
a direct comparison will serve to show that much more light comes to the
eye from the diamond surface than from the sapphire surface. The image
of the light filament, as seen from the diamond, is much keener than as
seen from the sapphire. The same disparity would exist between the
diamond and almost any other stone. Zircon comes nearest to having
adamantine luster of any of the other gems. The green garnet that is
called "olivine" in the trade also approaches diamond in luster, hence
the name "demantoid," or diamond like, sometimes applied to it.

VITREOUS LUSTER. The other stones nearly all have what is called
_vitreous_ luster (literally, glass like), yet owing to difference of
hardness, and consequent minute differences in fineness of surface
finish, the keenness of this vitreous luster varies slightly in
different stones, and a trained eye can obtain clues to the identity of
certain stones by means of a consideration of the luster. Garnets, for
example, being harder than glass, take a keener polish, and a glance at
a doublet (of which the hard top is usually garnet and the base of
glass) will show that the light is better reflected from the garnet part
of the top slope than from the glass part. This use of luster affords
the quickest and surest means of detecting a doublet. One can even tell
a doublet inside a show window, although the observer be outside on the
sidewalk, by moving to a position such that a reflection from the top
slope of the stone is to be had. When a doublet has a complete garnet
top no such direct comparison can be had, but by viewing first the top
luster, and then the back luster, in rapid succession, one can tell
whether or not the stone is a doublet.

OILY LUSTER. Certain stones, notably the peridot (or chrysolite) and the
hessonite (or cinnamon stone), have an oily luster. This is possibly due
to reflection of light that has penetrated the surface slightly and then
been reflected from disturbed layers beneath the surface. At any rate,
the difference in luster may be made use of by those who have trained
their eyes to appreciate it. Much practice will be needed before one can
expect to tell at a glance when he