Diamonds are one of the few products that simply cannot be branded. Even though there are different cuts, different grades, and different values placed on each and every diamond in existence, no diamond is any specific brand just as gold is not a specific brand.
Branding is actually based on who owns the diamond. For instance, if DeBeers owns the diamond, it is a DeBeers Diamond but it is still just a diamond. If the diamond was cut by a specific well known cutter, then it might be branded in that way as well but it usually isnt. It is still branded based on who owns it at the time. So basically, when it comes down to it diamond brands mean absolutely nothing at all.
Do not allow a jeweler to try to talk you into paying an exorbitant price on a diamond because it is a specific brand. This is a bit of trickery used by unscrupulous jewelers when they know that they are dealing with people who dont know much about diamonds. Remember that diamonds are not actually branded unless mother nature has her own...