Interesting!

What shape was the diamond?

ah2bqat, was this an older piece of jewelry? I can't remember where I learned of this – it has been quite awhile – but it is my understanding that colored gemstones (esp. ruby, sapphire) have been mounted under diamonds in rings as a subtle way to affect the perceived color of the diamond when viewed from above. I wish I could remember where I sourced this info. I've never seen it done in person, so of course, I have no idea how viable this type of practice is. Could be completely ridiculous. But, I do remember running across this somewhere. Anyway, without seeing a pic of what you were looking at it is difficult to tell exactly what is going on. Perhaps it is just a little design surprise...?

Trinkette, I think you were right the first time - it's to make a diamond throw blue light.  I wonder what the face would look like.  Shame it didn't have one of those to go with the side shot.

Maybe we could park some bitty sapphires behind some tiny diamonds and pretend they're BLOOO?

Nothing new there? 

Foxylady 

For what it is worth, SOMEONE out there thinks the sapphire-under-diamond method works as a way to give the impression the diamond radiates color. Currently, there is at least one jewelry classified on the internet that advertises a new diamond solitaire ring, quite modern in style, actually,  with the diamond mounted above two blue sapphires. Part of the description for the piece reads: "The mounting contains two sapphires set directly under the solitaire, which causes the diamond to emit natural blue tones with light reflection." 

Sorry, ah2bqat, I didn't get back to you on this. The web photo I saw was angled only from the side... 

The designer probably just thought it was pretty? Sapphires and other precious gemstones are usually set as the center stone or as accent stones, mounted on either side of the center stone. A sapphire set under a diamond is an unusual design but I see no problems with it.