I loved Diana's ring in 1981 but now.... It does look rather bulbous.
I believe Fergies ring was originally a different stone, an emerald I think? Can't be sure. Anyway, she asked Andrew to have the stone replaced for a Ruby. I can't remember the story now, perhaps she just didn't like the colour of the original stone and he hadnt realised.
She really loved it, she wore it for years and years after the divorce.
I believe the Queen Mother's was originally a sapphire (I think a cabachon?) , but for some reason it fell out of favour and there are hardly any pictures of her wearing it and none are detailed so you can't get a good look at it. Could be she hated it from the get go.