Gemstone Properties and Cutting Angles of Common Faceting Materials
The critical angle is the angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected. Critical angle can be calculated as the inverse Sine of 1 divided by n (the refractive index) of a gemstone. Refractive index (R.I.) of a substance is the speed of light within a substance compared to the speed of light in air. Remember the Critical Angle is what you want to be aware of when cutting the pavilion facets. The most brilliant gemstones are those whose pavilion facets (bottom of the stone) act like mirrors, reflecting light back up through the table and crown (top of stone) to the eye. Gemstones whose pavilion facets behave as windows, passing light out the bottom of the stone, look dull and less brilliant.