This brightness is light being bounced around inside the gemstone and being reflected back to your eye. You cannot see through a well cut gemstone because almost all light is being reflected back towards your eyes. Windows are some thing that you are meant to look through and these are better left as windshields in you car or as windows in your home. If you can see through a gemstone looking from the table down towards the culet, the point on the bottom, it has a window. If you can see through a gemstone it means that light is passing through it, along with color and brilliance, it's gone.
Windows weaken the intensity of color and severely affect brilliance.
Gemstones with windows are not desirable, they lack beauty and brilliance. Confidence Confidence is an additional quality that I have added here. You need to have confidence and trust with those whom you are dealing with. Ask questions and expect answers to your questions regarding a dealers expertise in gemstones or ethics. Weak or faded colors have less value.