The most important aspect about how bright a diamond is going to be is how well it is cut. We don't mean it's shape but instead how exacting, to perfect proportions the stone is cut. If a diamond is not cut to extremely precise specifications, it will "leak" light out the bottom. It will also let too much light in from the bottom so that you see what is under the stone. Sometimes this is oil from your skin, soap, darkness under the mounting, the mounting itself, or even your finger. In almost all cases this "windowing" is very undesirable, and does not show up in a well cut diamond.
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Why would anyone cut that is not to these exacting proportions? To save weight. When you cut a diamond you usually "lose" approximately half the original weight of the stone. If you "swindle" a diamond, you can cut a larger finished diamond from the original stone. BUT you suffer greatly in the brilliance and fire (dispersion) when you do that. Dispersion is the individual color flashes that give diamonds part of the look a diamond has instead a piece of glass. Swindling stones allows the unknowing or unscrupulous salespeople to sell you a much larger diamond for less. Unfortunately, many if not most, diamonds are swindled from a slight extent to an extreme extent.
Above is an example of how two different stones can be cut from the same piece of diamond rough. The top one is an example of how a ideal cut stone (we will say a one carat diamond ) would be cut from this rough. You would cut off the top and make a small stone from that piece, but you would "lose" all the rest of the material around the finished diamond.
In the bottom example you could cut the top too shallow, the girdle too thick, the bottom too deep and get a 1.40 carat diamond out of the bottom and another swindled large stone out of the top. It doesn't take the proverbial rocket scientist to figure out if you want a blasé diamond that is bright only when it is extremely clean, you can get a "bargain" with a swindled stone. Unfortunately a person can not always keep your diamonds that clean unless you clean them many times a day. This is the first area that most bargain diamond sales people usually swindle people. If the diamond "leaks" light it is not as bright.