Cycles Glass BSDF on a Diamond viewport problem I've got an issue with my glass viewport render.
It creates some kind of a hole in that object, which looks bad. In comparison to the tutorial, with simplest nodes and everything super basic, with the same render and light paths settings, it's just totally unlike.
Of course the shape is a little bit different, but I guess it shouldn't look like that anyways..
Do you please have any ideas how to fix that?
From my experience in Blender, a more complex diamond mesh gives the light more directions to scatter, which will increase the number of refraction effects you can detect on the surface.
In jeweler's terms, the cut of the diamond influences the clarity, sparkle, and brilliance.  Sometimes these are at odds with each other.

Also, increasing the number of Glossy and Total bounces under Light Paths will get rid of these dark areas.