The
luster of gems is one of their important and distinctive characters.
Not only does it form one of the easiest means of distinguishing gems,
but it is also one of the most reliable characters sought by those
experts who depend for their determinations of gems on ocular
examination alone. One familiar with the luster of quartz, as compared
with that of diamond, for example, is in little danger of confusing the
two, for the luster of one can be recognized as adamantine, that of the
other as vitreous. The luster of a gem is produced by the light which
it reflects back to the eye, and this may vary in quantity and quality
with the nature of the surface. Since the latter is largely the result
of the molecular structure of the mineral, it follows that different
species will have distinctive luster. The terms used to describe the
different kinds of luster are derived from that afforded by some
well-known object. Thus adamantine luster means the luster of the
diamond; vitreous luster, the luster of broken glass; oily luster, the
luster of oil; waxy luster, the luster of wax; resinous luster, the
luster of resins; pearly luster, the luster of pearl; silky luster, the
luster of silk; and metallic luster, the luster of shining metals.
Of
the above kinds of luster, the vitreous is the most common among gems,
being displayed by quartz, topaz, beryl, tourmaline, sapphire, and many
others. The adamantine luster belongs almost exclusively to the
diamond, although it is displayed to some extent by sphene and
colorless zircon and is suggested by some sapphire. It is
characteristic of minerals of a high index of refraction. Metallic
luster is strictly possessed only by opaque minerals, and hence among
gems is confined to pyrite and hematite. The luster of turquois is of
the waxy order. Essonite displays a somewhat resinous luster, and
chrysolite an oily one. Pearly luster is best seen in the pearl, but is
also illustrated by moonstone and opal. Tiger eye and cat's eye afford
examples of silky luster.
