I have looked again at the video of the christening of Prince Harry and, although at times the central stone in QEQM's brooch catches the light, at no time does it look like the emerald. The central stone seems not only opaque but to absorb light.
When I initially commented in September, I assumed that the stone was an amethyst but an amethyst would refract light and show some purple tones. This stone does not. That is, in my opinion, clearly highlighted by the video where we can see movement.
I wonder if the dark stone, which some of us are intrigued by, could be either a black sapphire or a black diamond? Both stones absorb light.
Having said that, I have never personally seen a large black diamond, only small ones. A black spinel is another possibility.
According to the press, Mrs Greville owned black diamonds, although I have never seen press reports of her wearing black diamonds to various functions.