Just a quick question about the "shadow of observer's head" phenomenon. While a deeply cut diamond is clearly dark in the centre, I've noticed that most diamonds (especially the well-cut ones) display a dark centre when looking directly at it (which is different from the deep-dark effect).

Great observation.

I hope Garry (Cut Nut) will share his thoughts on this issue since he is currently studying this effect.

Bruce Harding had written about it 25 years ago: http://www.cutstudy.com/cut/english/faceting/ and particularly: http://www.cutstudy.com/cut/english/faceting/f1.htm#dead

I've noticed that at times, the centre of the stone is completely white. Actually, the dark reflection is most obvious when in the car (with a dark roof overhead).

The darkness in the center increases when head distance decreases. However for ideal cut diamond it also creates a pleasant contrast and allows you to see more fire.

Diamonds with deep pavilion have very dark center though - nail-head effect. This image is simulated for a diamond with 44 degrees pavilion and 33 degrees crown angles in the jewelry store environment and 12" viewing distance.