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LUSTRE
The luster or brilliance of
transparent gems is caused by light reflecting from the stone's surface. The
smoother and more highly polished the surface is, the greater the luster
will be. High light refractivity of a gem will cause greater luster as well.
The most intensive luster is seen in the highest refractive indices,
diamond, zircon, and rutile, and is known as an adamantine luster. Hematite
produces a metallic luster, even though it is not transparent. Most
gemstones have a vitreous or glassy luster, but there are other types of
lusters, including resinous (amber), greasy (serpentine), waxy (turquoise),
pearly (rhodonite), and silky (tiger's eye) The overall appearance of a gemstone, its "lustre", is determined by
the way light is reflected from its surface. Gemmologists use a variety of terms to describe lustre and its degree of
intensity. "Splendent" means that the stone reflects light like a mirror;
but if little light is reflected, the lustre may be described as "earthy"
or "dull". Stones with a lustre comparable to diamond are described as
"adamantine", and are the most desirable. In fact, most transparent, faceted
gems have a glass-like, "vitreous" lustre; the precious metals have a
"metallic" lustre; and organic gems show a range, from "resinous" to
"pearly" and "waxy". Some gemstone species vary in their lustre: garnets,
for example, range from the resinous hessonite garnet to the adamantine
lustre of demantoid garnet. Rough lazulite and howlite have a dull, earthy
lustre, which is vitreous after polishing
INTERFERENCE
Interference is an optical property caused by the
reflection of light off structures within a gemstone. This internal
reflection gives a play of colour. In some stones it will produce the full
range of the spectral colours; in others just one colour may predominate. In
opal, interference occurs because of the structure of the stone itself—
spheres arranged in regular three-dimensional patterns. This produces the
rainbow effect called iridescence, also shown by a number of other gems such
as hematite, labradorite, and iris quartz. In moonstone feldspar,
interference at the junctions of its internal layers (thin, alternating
layers of different types of feldspar) produces a shimmering effect just
below the surface of the stone, known as adular-escence, opalescence, or a
schiller (sheen).
Natural Gemstones |
Metallic Lustre
Adamantine Lustre
Vitreous Lustre
Greasy Lustre
Resinous Lustre
Silky Lustre
Waxy Lustre |
