It seems to be very elusive the "Cushion Brilliant" and most manufactures are clueless when it comes to producing an ideal cushion.  EGL has no clue what or how to call a true Cushion Brilliant and GIA is being very conservative on the definition.  As a true Cushion Brilliant the facets on the pavilion should reach the girdle hence the "Brilliant" like a Round bottom, and a modified has a radiant cut that the pavilion facets will not reach the girdle.  I’ve seen more than a dozen varieties of “Cushion Cuts” and recently an Israeli cutter created what I would call a "Hybrid" cut; that has the four pavilion facets reaching the girdle and the modified facets in between.  The face-up is inconsistent as they can appear like a modified or brilliant and originally GIA called these Brilliants, which as a Cushion purist upset me. The reason why is that “Cushion Brilliants” are really popular and now cutters can cut a "cushion brilliant" diamond that will retain the extra weight and look of a modified and still get the premium for that cut. Though recently GIA started calling these “Hybrids” modified which probably pissed off a lot of Israeli cutters but certainly opened up the debate on what is a true “Cushion Brilliant”.  A true "Cushion Brilliant" should not window at angles(loss of life at angles, usually appears to have four quadrants) or appear like shattered glass. 

This is an email I sent to an Israeli Diamond cutter who tried to explain why they do not cut true Brilliants: Though you are right they loose more weight, though when a 1ct is cut properly it should spread 6mm on average and most are cut to heavy.  Once they establish ideal parameters cutters will not be able to sell the heavy junk they have been cutting.  I like it to sparkle like a round with small flashes like a princess though it appears its true size, and unfortunately only a true brilliant with a round bottom will satisfy me.  Though that cut will always acquire a premium as it does loose the weight like a round and is almost impossible to find, as most cutters have that attitude you described.  Sorry for the long email I just love my Cushions and yes if cut to traditional standards GIA will consider them "Brilliants".

- Troy Lob, Graduate Gemologist (GIA), Vice President, Liberty Diamonds