You're quite welcome. And yes. It's fascinating.
I'm not sure the photo properly shows the nuance, but one example of 'special' D on left, natural D on right.
No and yes. I never saw variation in material transparency due to angle of observation. I was only looking from top-down assessing performance qualities. In slight cases it presents like double refraction (think faceted calcite). Beautiful performance when well-cut but slightly off from diamond. In more pronounced cases there's muting, resembling natural diamond where one side didn't take a good polish. The first reaction is to grab a microfiber cloth and "fix it."
I'd start with this... No diamond is "easy" to polish. It's hard. It's stubborn. It fights. It doesn't want to be polished. It was perfectly happy like it was.
That's the reason the earliest cuts essentially followed the outline of octahedron and dodecahedron and twinned/flat crystals (rose cut for the latter).
Credit: Erstwhile Jewelry for the nice graphic.
Yes, Argyle rough has more twinning and structural defects than other primary souces. That's why we get more pink and red FCD from Argyle, by the way.
With all that said, for better or worse, our plans, tech and tools were developed to follow the essential shape of natural octahedron or dodecahedron. So when we introduce material which grew in vertical layers there's a learning curve. It may not be physically 'harder' but it's a new frontier.
You cause me to wonder though. It may be easier to polish CVD that isn't annealed to bleach the color (left) than HPHT-annealed material which goes through another phase (the two cubes top right). I can ask about that.
Here's a decent explanation and animation of CVD growth.
And I love this video. About 1 minute in you'll see the intense process used to take a rough cube to a more viable semi-polished state where it may find its way forward with traditional girdling, blocking and polishing tools.
https://www.adadiamonds.com/cvdgrowth
Can you imagine leaving so much natural rough on the cutting room floor?
Of course if the earth had given us cubed rough the round diamond wouldn't have evolved. Too much waste. Preserving shape and weight would mean a world of shallow emerald cuts, maybe with some pesky radiant and princess concepts popping up here and there if someone thought kite-shaped facets would be groovy.
Interesting footnote: Because of its layered growth, it's not interesting to cut steep-deep from CVD.
Cue applause.