No specific profile was used. To achieve this look the tiles on the primary wall are overlaid in front of the tiles going into the niche...the other critical component here is to make the openings the exact size of the tiles, so you don't have irregularly shaped tiles around the box.

The advantage to stone is that the edges can look finished once the tile is all sealed--with porcelains you will usually need to use some kind of profile because even through-body porcelain can sometimes appear unfinished on the edges. We use this method a lot for a super clean modern finish, as Joel accomplished here. (Loving that rug, by the way)

thank you JK Design & StoneMar for the valuable information!