DIAMOND

WHAT IS IMPORTANT ABOUT Index of Refraction, Birefringence and Dispersion?

CALCITE

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The Index of Refraction, Birefringence and Dispersion are somewhat exotic properties for ordinary rockhounds, but they are consistent properties in that minerals never stray far from their known values. This makes them invaluable tools to mineralogists for identifying and studying minerals.

The index of refraction is the geometric ratio of the angle at which light comes to the crystal (called the angle of incidence) by the angle at which light is bent as it enters a crystal (called the angle of refraction). Metallic minerals do not have an index of refraction because they do not allow light to enter the crystal in the first place. The laws of refraction (called Snell's Laws) were laid down by Willebrod Snellius in 1621 and he proposed the following formula:

(sine i)/(sine r) = n

Where i is the angle of incidence and r is the angle of refraction and n is the index of refraction. It turns out that this ratio n is also the ratio of the speed of light in air to the speed of light in the crystal. This relationship shows the impact of density or specific gravity to the index of refraction in that the greater the density the slower the speed of light. But density is not the only impact to the index of refraction (if it were, we could used index of refraction to measure density and we can't do that, directly anyway) as chemistry and structure play an important part too. Generally the index of refraction for minerals falls between 1.4 to 2.0 with a few exceptional mineral exceeding 2.5.

The symmetry of the crystal has interesting impacts to the index of refraction. Isometric and amorphous minerals have essentially the same structure or lack there of, in all directions and so have only one index of refraction and are called isotropic minerals. But hexagonal, trigonal and tetragonal minerals have a different structure along their primary axes than they do in all other directions and for this reason they have two indices of refraction; one along the primary axis and one for every other direction. These minerals are called uniaxial minerals for their one unique direction. Orthorhombic, monoclinic and triclinic minerals have two planes of equal refractive indices and are called biaxial.

Some Common Gemstones and Minerals and Their Index of Refraction Range

MINERAL:	IR:	MINERAL:	IR:	MINERAL:	IR:
Almandine	1.830	Andradite	1.887	Apatite	1.624- 1.667
Aragonite	1.530 - 1.686	Barite	1.636 - 1.648	Beryl	1.565 - 1.598
Calcite	1.486 - 1.740	Cerussite	1.804 - 2.079	Chrysoberyl	1.746 - 1.756
Corundum	1.759 - 1.772	Diamond	2.418	Fluorite	1.434
Grossularite	1.734	Gypsum	1.519 - 1.531	Halite	1.544
Microcline	1.514 - 1.539	Olivine	1.63 - 1.88	Opal	1.41 - 1.46
Quartz	1.544 - 1.553	Rhodochrosite	1.597 - 1.816	Rutile	2.605 - 2.901
Scapolite	1.546 - 1.600	Sodalite	1.483 - 1.487	Spessartine	1.800
Sphalerite	2.369	Sphene	1.843 - 2.110	Spinel	1.719
Topaz	1.606 - 1.638	Tourmaline	1.635 - 1.675	Zircon	1.923 - 2.015

The average collector might be able to use the index of refraction to gauge a mineral's sparkle and generally gemstones that have a high index of refraction are desired above others. Gemstones that have an index or refraction near 2.0 or higher are considered good refractive stones. Observe the different gemstones above for the ones with high index of refractions and those with low values. The following properties of birefringence and dispersion are closely related to the index of refraction.

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BIREFRINGENCE AND DOUBLE REFRACTION

The difference between the highest and lowest index of refraction in a mineral is called the birefringence. The birefringence is generally low in most minerals but is high for carbonates and a few other minerals. Calcite has one of the highest degrees of birefringence and this causes the phenomenon of double refraction. Double refraction occurs when a ray of light enters the calcite crystal and due to calcite's high birefringence, the ray is split into beams, one very fast and one very slow; relatively that is. As these two beams exit the crystal, they are bent into two different angles (the angles of refraction) because the angle is directly affected by the speed of the beams. A person viewing into the crystal will see two images ..... of everything. The best way to view the double refraction is by placing the crystal on a straight line or printed word (the result will be two lines or two words). There is only one direction that the beams are both the same speed and that is parallel to the C-axis or primary trigonal axis. Rotation of the crystal will reveal the direction in the crystal that is parallel to the C-axis when the line or word becomes whole again. By contrast, the direction perpendicular to the C-axis will have the greatest separation.

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DISPERSION

Dispersion is more of a concern to gemologists than to mineralogists. It is a very important property to used to identify and qualify gemstones. Dispersion is another property that is affected by the index of refraction. The above discussion of refraction dealt with the refraction of only the same wavelength of light. But to make it more complex, refraction is affected by the wavelength as well. Blue light is bent more than green light which is bent more than red light. If dispersion in a mineral is low, than white light can travel through the mineral nearly unaffected and emerge as white light. But if dispersion is high, the white light will have its component wavelengths or colors dispersed through increasing refraction. This is what causes the flashes of color, called fire, in cut gemstones. Diamond is the champion of cut stones and has a high degree of dispersion or fire that is almost always unmatched by diamond simulants. Zircon, cubic zirconia and YAG all have high dispersions and are the popular diamond impostors of the day, although zircon is a lovely gemstone in its own right. Dispersion is the reason we have rainbows and why a glass prism can separate light into its many colors.