This part is a little more involved -- but not much. Transmitted light is the portion of light that enters the material, or a diamond in this case. Again, reflected light does not enter the diamond, but bounces away at the same angle that it met the surface. Transmitted light is that which enters the diamond. Imagine yourself skipping stones off the surface of a perfectly calm lake. You understand that in order to actually make the stone reflect off the surface, the angle of incidence must be fairly slight, say 2° to 10°. If you heaved another stone at the surface at say 30° (from further up the embankment), the chances of it skipping are greatly reduced. At 60° the stone will more than likely be swallowed into the blue water every time. At 90° above the surface, the stone would be virtually impossible to skip or reflect off the surface. It would be "transmitted" into the water every time. The same principle works with light. The slighter the angle of incidence, the more likely the light will reflect off the surface of the diamond. On the other hand, at 90°, almost all the light is transmitted into the diamond.
Lets get back to refraction for a moment. The light that is reflected off the surface of a diamond is lost forever. But the light that enters (transmitted), reflecting inside the diamond is part of what makes it beautiful. Since light is traveling in many different directions simultaneously the necessary amount to make a well cut diamond come to life can be very little. All light that is transmitted is also refracted or bent into the array of spectral colors, which then reflects inside the diamond's surfaces, and is eventually released as the rich dispersion that sets diamond apart from other gemstones.