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Optical Properties

Refractive index  


Physical Properties

Synthetic Gems



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 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





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