Light is the catalyst that makes a gemstone beautiful. For purposes of our discussion, light will be understood to be white light or a mix of all colors of the spectrum.
A gemstone’s optical properties make perfect use of light. The optical properties are hue / color, refractive index (R.I.), dispersion, absorption, transparency, luster and pleichroism.
Light striking a gem surface
A ray of light which strikes the gem surface is called the incident ray.
The ray of light which leaves the gem surface is called the reflected ray.
A line perpendicular to the surface can be imagined at the point of reflection and is called a normal .. an imaginary line that runs perpendicular to the gem’s surface.
The angle between the incident ray and the normal is called the angle of incidence “AI“. The angle between the reflected ray and the normal is called the angle of reflection “AR“. Notice that the angle of incidence = the angle of reflection.
Reflection vs. Refraction in Gems
If light does not reflect off a gem surface it will refract into the gem material. Refraction is the bending of the path of a light wave as it passes across the boundary separating two media. Refraction is caused by the change in speed when a wave changes mediums.
If a light wave passes from a medium in which it travels slow (relatively speaking) into a medium in which it travels fast, then the light wave will refract away from the normal. Conversely, if a light wave passes from a medium in which it travels fast (relatively speaking) into a medium in which it travels slow, then the light wave will refract towards the normal.
Refractive Index aka “R. I.”
Speed of light in air [V1] – 300,000 km/sec
Speed of light in ruby [V2] – 169,500 km/sec
“R. I.” = V1/V2 = 300,000/169,500 = 1.77
Critical Angle in Minerals
Each gem material has a unique critical angle. That critical angle is determined by the gem material’s “R. I.” or “refractive index” Critical angle aka “C. A.“, is what determines whether a given incidence ray will reflect or refract.
Light striking the gem at an angle within the “C. A.” cone (cone of acceptance) will refract into the gem. Light striking the gem at an angle outside the C.A. cone will be reflected
“C. A.” is defined as – the angle of incidence which provides an angle of refraction of 90-degrees. Note that the critical angle is an angle of incidence value. Critical angle is represented by the “C. A.” cone in the illustration. The formula for “C. A.“: sin C.A. = 1/R.I.
Dispersion of Light
Dispersion is the separation of white light into its spectral components. The prismatic effect of the crown facets on a gemstone create the phenomena we call spectral dispersion.
Upon entering certain crystals, light will be divided into two rays. Double refraction is present in all gemstones except Opal and those belonging to the Isometric (cubic) crystal system.
Evidence of double refraction can be observed in finished gemstones by the doubling of facet junction in the pavilion when observed through the table.
Another evidence of double refraction can be seen under polarized light. And the phenomenon can sometimes result in the display of primary and secondary colors when observing a gem from off the primary axis.
Luster in Gemstones
Luster is one of the most important characteristics of gem materials. Luster can be discussed as either external or internal .. or both.
External luster refers to incident light reflected off a gem surface. The quality of external luster depends on the quality of finish on the facets and the hardness of the gem material.
Internal luster refers to light that has entered a gemstone, is reflected internally, then returns to the eye. The quality of internal luster depends on R.I. and gem proportions .. and is responsible for much of the beauty of a gemstone