Yup, the official version is that a tray full of rings was brought to the Palace for Diana, and she chose the biggest sapphire in it. The ring was alreeady photographed for Garrard's Christmas Catalog and it had already gone to print, so Diana got her ring, and then all the UK saw what it had cost. I saw that on one of the many specials they aired on TV as part of the royal wedding.
As much as I like Camilla's ring (EC perfection!), it is GRAY! While I love warm stones, I like them to tint towards yellow, not gray. It does look like she baked it a little too much.
That's how I remember it, borismom. I must've seen every TV special that aired about Diana when I was a teenager. I was enamored with the "fairy tale" princess-to-be.
But anyway, I think I saw a fairly recent segment on the Today show with that story of Garrard's Christmas catalog and Diana choosing it from a tray that was brought to her. It aired around the time William and Kate got engaged.