2) Physical properties of the gem material: there are many properties of gem materials that help determine which specific design is most appropriate. While a complete explanation of this aspect of design selection is beyond the scope of this tutorial, some general points can be made. The single most important physical property of any gem to be cut is its refractive index, which basically determines whether light will be reflected within the gemstone or transmitted through it for a given set of facet angles. If this property isn't carefully taken into account, a design might be chosen that allows light to escape through the pavilion of the stone rather than being reflected back through the crown. When pavilion angles are too shallow for the gem material in question, light passes right through the bottom, and the resulting effect is called "windowing." The images below, generated using a specialized computer program for simulating gemstone behavior, illustrate this concept. Compared are two different materials (diamond and quartz) cut in the exact same way.

		See how a ray of light behaves in a brilliant-cut diamond? Notice that most of the light returns through the crown.
		A photo-realistic simulation of the diamond shown above. The stone shows good brilliance.
		See how a ray of light behaves in a piece of quartz cut in the exact same way? Notice how much light is leaving via the pavilion.
		Here is a photo-realistic simulation of the quartz. Notice the large window in the center.