This type of diamond setting is called a pie diamond setting. This is much like the tongue and groove technique of settings square diamonds together to give the illusion of a bigger square diamond. These pie diamonds similarly use a tongue and groove technique so that diamonds don't slide against each other but remain riveted in place without the use of surface prongs except on the circumfrence.
The cost savings can be substantial for the look that you are getting. An oval shaped diamond with a total carat weight of 3 carats will cost at least $8000-$12,000 per carat at retail. That similar look can be achieved by putting together four pie slices that are half to a third of a carat each and will be priced at $1200 per carat at retail.
Granted, the cuts have to be quite precise so that the fit is good and the oval shape looks convincing. So when you add $3600 for the price of all four slices to make up your 3 carat you're talking about substantial savings. Not all women will go for this look. Many think it looks like a cheap white gold diamond ring because the ridges look kind of obvious and so for them it's not a great deal. They would just as soon get a white gold cubic zirconia ring instead.
The other problem with diamond pie rings is the durability. Remember that the stone is actually in four pieces and they can come apart if the integrity of the mounting is lost. The mounting itself like most 14k white gold wedding rings is not a bezel set it is a prong set which is part of the attraction in the first place. So if one of those prongs gets bent then all four pieces could fall out.
This type of diamond look is great for diamond rings as long as the wearer can be careful as to how the ring is worn. Women who are hard wearing on their jewelry should avoid these types of diamond fashion rings. Instead, they can get a nice pair of diamond stud earrings using the same pie concept except that they don't have to worry that the setting has high chances of getting damaged. They can get a pair of diamond studs that face up almost like a two carat total weight pair but with the cost of a half carat total weight pair.