What Makes a Diamond Shine?

Diamonds shine because their special cut creates a state called "total internal reflection." That means that the light entering from the top will reflect off two facets before returning back out the top. This special effect is created by skillful diamond cutting and is based on diamond's high refractive index. This high refractive index means that the direction of light bends a lot as it travels from air to diamond or from diamond to air. Because of this bend, when light travelling through the crystal lattice meets a face at an oblique angle, it will bounce off rather than penetrate through it to the outside. Generally light that enters a properly cut diamond from the top will bounce off two faces before returning to exit out the top. At this point the light is heading straight into a top facet (head on) and will emerge rather than bounce off of it as it did when it met the earlier facets. The process is so delicate because if the diamond's pavilion is cut too deep, the light will leave through one of the lower facets. If the pavilion is cut too shallow, the light will exit through a side facet. Light can enter and leave the crystal in so many ways and bounce around in so many different directions that getting the best effect is a special art. For this reason, only the most experienced diamond cutters are called upon to cut the largest and most expensive stones. ********************************************************** The refractive index of diamond is pretty high (2.417) and is also dispersive (coefficient is 0.044). Due to this fact, diamond is an important application in optics. Consider an ideal cut diamond. I explain according to the figure below. When the light is incident at an angle 1, it refracts inside and travels through the lattice. At the surface which separates air & diamond media, the incident angle 2 is very well above the critical angle (ca) and simultaneously (3 & 4) the reflection takes place at different surfaces of the diamond. Finally, the light refracts out. But diamond on the other hand has a really high refractive index (≈2.4) and because of that the critical angle for total internal reflection to occur is much smaller. So a greater percentage of the incident light gets internally reflected several times before it emerges from the diamond, making the diamond look really shiny.