The visual appeal and characteristics of a gemstone are determined by several factors including: brilliance (sparkle), color, fire (light dispersion), and luster (surface reflectiveness). A stone's brilliance, fire, and to a lesser extent luster, are influenced by the type of cut used.
The faceting of a gemstone will affect how light will behave as it passes through the outside surface into the interior of the gem. Light can either be reflected off a surface affecting the luster of the stone, or pass through the surface into the mineral and be refracted, scattered, and dispersed as the light exits the stone. As a light beam passes through a gem it is bent or refracted before it exits the crystal. The light beam is also broken into its component parts (dispersion) causing the effect known as "fire". This dispersion widens the beam to the point that the observer can see the full visible spectrum of the beam from red to violet, simulating a rainbow effect. As the stone is moved, the refraction and reflection points of the facets change showing the stone's scintillation or "play of color."
Faceting a Gemstone - The Critical Angle
The "critical angle" at which a light beam intersects with a given series of facets is the angle at which total "internal reflection" is achieved. Light traveling through the interior of the stone will eventually intersect the stone's outer surface (from the inside). If the light intersects within the "critical angle" it will exit the stone. If the light intersects outside of the "critical angle", it will be internally reflected and lost.
The "critical angle" calculation is used to determine how facets should be placed in relation to each-other in order to control the path of light within a gemstone. In a properly faceted gemstone, the light should be reflected internally off the pavilion facets, and pass through the crown facets to achieve maximum brilliance, sparkle and scintillation (below). On the other hand, the light path shoud not escape through the pavilion.
Refractive Index
When light passes from one density or type of material (air) into another (gemstone), it is bent or "refracted". The amount, or degree, that the beam of light is bent will be determined by the density difference between the air and the gem. The measurement used to quantify the amount that the light is bent is the "refractive index" (RI).
There are two factors when calculating a gem's refractive index: 'angle of incident' and 'refractive angle'. The 'Incident Angle' is the angle of the approaching light as it intersects with the stone's surface. The "refractive angle" is the altered angle of the light as it passes through the stone. The Refractive Index is the ratio of difference between the two angles. Each material has its own unique density and Refractive Index.
Double Refraction in Anisotropic Gems
Certain anisotropic crystals exhibit a phenomenon known as 'double refraction', where the incident light is split into two separate rays, each with different refractive indices and velocities. Crystals that belong to very symmetrical crystal systems (cubic, hexagonal) are always singly refractive, and those that belong to less symmetrical crystal systems (monoclinic, triclinic) tend to have a higher occurrence of double refraction [2]. The mineral calcite (above, left) has two different refractive indices (birefringence) of 1.490 and 1.660 exhibiting strong double refraction.
Reflection vs Refraction
In order to understand why a gem is faceted or cut en cabochon, it is important to understand how light will behave once it passes into a gemstone or is reflected off its surface. The table below shows several different types of optical effects that are the result of light being refracted (1. spectral dispersion, 2. double refraction) and/or reflected (3. light scattering) from the gemstone.
1. Pseudochromatic Coloration: Caused from optics effects created by spectral dispersion:
2. Double Refraction: Caused by incident light being split into two separate rays:
- Birefringence: Calcite, Moissanite, Zircon
3. Pseudochromatic Coloration: Caused from optics effects created by light scattering:
- Chatoyancy: Alexandrite, Tiger's Eye
- Asterism: Garnet, Star Ruby, Star Sapphire
- Luster: Pearl, Talc, Gypsum
- Aventurescence: Sunstone, Aventurine Quartz
- Adularescence: Moonstone (blue), Opal (milky white)
Refractive Index of Common Gemstones
Refractive Index of Metals
- Copper: 1.100
- Gold: 0.470
- Nickel: 1.080
- Platinum: 2.330
- Silver: 0.180
- Titanium: 2.160
