Plot/Story
The story is fitting such complex characters and when you read the manga you truly feel that the characters are the way they are because of their life experience. Even though in my personal opinion, the author has first design the characters personalities and then the fitting story.

Characters
The personality of the characters: many of them are shown to have both good and evil in them: Ray, Azura, Solomon, and Mr. Tassel. For example: Ray is very cruel to Florian in his search for a diamond just because of its beauty and yet he risks his life to save Florian and is prepared to go against Azura (his blood brother) for that.

Drawing Style
The drawing style is one of the reasons why this manga is realistic.
It's a factor that helps with the creation of the bond between author/artist and reader. It shows the author's capability to make the world she created comes to life.
Enjoyment
With much too few exceptions, in general when you read a manga you have the feeling that it is made following a certain recipe and thus its "charm" Is diminished to minimum, if not gone, no mater how well it is done. Well I didn't have this feeling with "Gorgeous Carat - Kurayami no Bitoku " and that's because it is realistic. The whole universe of this manga comes to life through the reader's experience and the connection between him and the author.
Reading it was such a moving, amazing experience.

Overall
Thanks to the characters, the story the drawing style, the author/artist managed to connect with the reader and the world that was created "comes to life" through the feelings one has while reading this manga.that's why it is true masterpiece.