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The refractive index of a gemstone provides the single most important piece of information to a gemmologist seeking to identify an unknown stone. It is a constant that is measurable to four significant figures (i.e 3 decimal points) and can allow gems to be distinguished even when their R.I's differ only very slightly.
The bending of light when it passes from a rarer medium (Air) into a denser medium (Gemstone).
Light passing through a substance is bent from its original path but emerges as a single ray. Only occurs in gem minerals belonging to the cubic crystal system or amorphous materials.
Light passing through a substance is split into two rays, which travel at different velocities causing differing amounts of refraction. Occurs in gem minerals belonging to all other crystal systems.
Example: Doubling of the back facets as seen in either Zircon or Peridot.
|Velocity of light in air Velocity of light in a gemstone
|186,000 miles per second 77,000 miles per second
In 1621, W Snell, a professor at Leyden University, discovered the
"Law of Refraction" which states:
When a specimen is immersed in a liquid having a similar R.I, the relief is low (i.e the edges tend to disappear). To approximate the R.I. of an unknown specimen, immerse the stone in one liquid after another until one is found in which it most completely disappears.
Caution: Avoid using porous stones in the above liquids (ie Opal, Turquoise, Chalcedony, Lapis Lazuli)
The refractometer is based on the principle of "Total Internal Reflection" which occurs as incident light rays strike at angles greater than the critical angle (when travelling from a denser medium into a rarer medium) and are reflected back into the denser medium.
It is an optical instrument arranged to show the critical angle of total internal reflection as a shadow edge, on a scale calibrated in refractive indices.
The name applies to the phenomenon which occurs when a ray of light travelling through a denser medium to a rarer medium at an angle greater than the critical angle suffers complete reflection back through a denser medium.
That angle where a ray of light, travelling from a denser medium to one less dense, is refracted at an angle of 90 degrees to the normal, that is it skims along the surface separating the two media. Any further increases of the light ray angle would cause the refracted ray to turn back into the first medium where it obeys the ordinary "Laws of Reflection".
Distant "Vision" for Cabochons
There are a number of ways of determining whether a gemstone is doubly refractive.
Doubly refractive stones will display two shadow edges when viewed through the eyepiece of the refractometer. By turning the stone carefully on the glass prism, maximum and minimum birefringence can be calculated by subtracting the lower shadow edge from the higher one. This can be a valuable piece of information to a gemmologist seeking to identify an unknown gemstone.
Anisotropic gemstones possess either one (uniaxial) or two (biaxial) directions along which light is not doubly refracted. These directions of single refraction are called "Optic axes".
Both amorphous and crystalline substances can be grouped under these three headings:
Isotropic : Cubic or amorphous.
Uniaxial : Tetragonal, hexagonal and trigonal.
Biaxial : Orthorhombic, monoclinic and triclinic.
This provides yet another valuable piece of information to the gemmologist.
Uniaxial: Show a fixed refractive index for the ordinary ray and a varying one for the extraordinary ray.
Biaxial: The R.I. of both rays or shadow edges vary.
Positive: The moving shadow edge has a higher R.I. than the stationary edge.
Negative: The moving shadow edge has a lower R.I. than the stationary edge.
Positive: If the higher edge moves more than halfway towards the lowest shadow edge.
Negative: If the lower edge moves more than halfway towards the highest reading.
It is sometimes sufficient simply to know whether a gem stone is singly or doubly refractive. For this uncomplicated test, the polariscope comes into its own.
If the stone is viewed along an "Optic" axis (a direction of single refraction) it will appear dark as it is turned.Some stones show "Anomalous Birefringence" caused by internal strain within the stone.
Examples: Spinel, Glass, Diamond.
The sine of the critical angle can be calculated using the following formula:
|Sine of critical angle =
|R.I. of the surrounding medium R.I. of gemstone
To determine the critical angle of a gem mineral in air:
|Sine of critical angle =
|1 R.I. of gemstone
|Diamond Sine of critical angle =
The angle itself can be derived from a set of trigonometric tables
Critical angle = Arc sine 0.413 = 24.26 degrees
|Quartz Sine of critical angle =
Critical angle = Arc sine 0.649 = 40.30 degrees
That if a ray of light travelling through a diamond strikes the pavilion facets at an angle greater than 24.26 degrees, it will be reflected back within the stone (Total internal reflection).
If is strikes the pavilion facets at an angle less than 24.26 degrees, it will not be reflected back into the stone.
To achieve "Total Internal Reflection", the lapidary must adjust the angles of the crown and pavilion facets so that the majority of the rays meet the interior faces of the pavilion facets at angles, to the normal, which are greater than the critical angle.
If the angles are wrong, the rays will pass out through the pavilion facets and the stone will appear dark.
It is also important that the rays reflected back from the pavilion facets meet the crown facets at angles less than the critical angle. If they fail to do this, they will undergo "Total Internal Reflection" again instead of being returned to the eye.