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Optical Properties of Gemstones

Color is the most important factor in determining the value of gemstones and its the most obvious visual feature, but in fact it is just one of many optical properties, all of which are dependent upon light.

Luminescence

Luminescence incorporates a gemstone´s ability to emit visible light in darkness when exposed to ultraviolet light (fluorescence, named after flourite, the predominant flourescent gemstone), and in the case of kunzite, to produce an afterglow which lingers after the light has ceased (phosphorescence).

Luster

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).

Pleochroism

Color changes which are evident when viewed from different angles in gemstones (iolite, tanzanite, alexandrite, andalusite) is called pleochroism. It is very important for the gem cutter to cut a pleochroic stone properly in order to show off the different colors. Ruby and sapphire have two color shades and are pleochroic; in ruby, for example, yellow-red and purplish-red, which distinguishes it from garnet and red spinel, which have no pleochroism. Iolite displays lavender-blue, gray, and pale yellow when viewed from different angles.

Dispersion

Dispersion is the separation of light into its separate spectral colors. Gemstones with the highest light refraction typically show the highest dispersion rate as well (rutile, sphene, diamond, zircon). This color dispersion or fire can be enhanced by a gem cutter if he uses an appropriate facetting style.

Refraction

light ray which hits the gemstone and passes into the stone. As it enters the denser medium of the gem, the light bends and the amount of bending or light refraction produces a measurable index (refractive index), which is often used to help identify a gemstone. When light hitting a gemstone splits into two rays traveling through the stone at different speeds and in different directions, the reaction is called birefringence or double refraction. This is seen uncommonly and in a variety of calcite called Iceland Spar as well as zircon, rutile, and sphene.

Refractive Index of Gemstones

GEMSTONE REFRACTIVE INDEX
1 Actinolite 1.614-1.642
2 Albite Feldspar 1.525-1.536
3 Almandite Garnet 1.790-1.790
4 Amber 1.540-1.540
5 Andalusite 1.634-1.643
6 Andradite Garnet (Demantoid) 1.875-1.875
7 Apatite 1.642-1.646
8 Benitoite 1.757-1.804
9 Beryl 1.577-1.583
10 Brazilianite 1.602-1.621
11 Calcite 1.486-1.658
12 Chalcedony 1.535-1.539
13 Chrysoberyl (Alexandrite) 1.746-1.755
14 Coral 1.486-1.658
15 Corundum (Ruby & Sapphire) 1.762-1.770
16 Cubic Zirconia 2.177-2.177
17 Danburite 1.630-1.636
18 Diamond 2.417-2.417
19 Diopside 1.675-1.701
20 Dioptase 1.655-1.708
21 Emerald 1.580-1.580
22 Fluorite 1.434-1.434
23 G.G.G. 2.030-2.030
24 Glass 1.440-1.900
25 Hematite 2.940-3.220
26 Idocrase 1.713-1.718
27 Iolite (Cordierite) 1.542-1.551
28 Ivory 1.540-1.540
29 Jadeite 1.660-1.680
30 Jet 1.660-1.660
31 Kornerupine 1.667-1.680
32 Kyanite 1.716-1.731
33 Labradorite  Feldspar 1.559-1.568
34 Lapis-Lazuli (Lazurite) 1.500-1.500
35 Malachite 1.660-1.910
36 Microcline Feldspar 1.559-1.568
37 Moldavite 1.500-1.500
38 Nephrite 1.606-1.632
39 Obsidian 1.500-1.500
40 Oligoclase Feldspar 1.539-1.547
41 Opal 1.450-1.450
42 Orthoclase Feldspar (Moonstone) 1.518-1.526
43 Peridot 1.654-1.690
44 Plastic 1.460-1.700
45 Pyrope Garnet 1.746-1.746
46 Quartz 1.544-1.553
47 Rhodochrosite 1.597-1.817
48 Rhodolite Garnet 1.760-1.760
49 Rhodonite 1.730-1.740
50 Scapolite 1.550-1.572
51 Serpentine 1.560-1.570
52 Shell 1.530-1.686
53 Sinhalite 1.668-1.707
54 Sodalite 1.483-1.483
55 Spessartite Garnet 1.810-1.810
56 Sphene 1.900-2.034
57 Spinel 1.718-1.718
58 Spodumene (Kunzite) 1.660-1.676
59 Strontium Titanate 2.409-2.409
60 Synthetic Corundum 1.762-1.770
61 Synthetic Emerald (Flux) 1.561-1.564
62 Synthetic Emerald (Hydro) 1.568-1.573
63 Synthetic Moissanite 2.650-2.650
64 Synthetic Rutile 2.616-2.903
65 Synthetic Spinel 1.730-1.730
66 TL Grossularite Garnet 1.720-1.720
67 Topaz (Blue,White) 1.619-1.627
68 Tourmaline 1.624-1.644
69 TP Grossularite Garnet (Hessonite) 1.740-1.740
70 Turquoise 1.610-1.650
71 Verdite 1.580-1.580
72 Y.A.G. 1.833-1.833
73 Zircon 1.925-1.984
74 Zoisite (Tanzanite) 1.691-1.704

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