Nyadriel
Neither. It's a matter of taste. Us ladies are particular in our own ways.
I have seen jewelry of sapphire or whatever gem 'enhanced' or surrounded by diamonds and I find them rather gaudy and not at all to my taste. Other gals like them that way.
Either way, it really cannot be translated in this virtual item to have diamonds mixed in with the other gems. And that is aside from that fact that each vault is meant to display the particular gem and not the diamonds with exception to the original.
You would have to have at least one of each to mix and match to your liking.
That's like saying the white of a canvas detracts from the paint upon it. The diamonds serve as a colorless "background" to enhance the main gemstone through contrast. The canvas is part of the painting, but it certainly isn't the focus of the viewer's attention.
Diamonds can only be the star when they are by themselves. Otherwise they are the second stringers because they don't have the color or vibrancy to stand out against the deep hues of the other precious gems.
As for impossible, it would require a certain sense of delicacy and taste to do properly, but it could be done.