We have seen this necklace at the Sun Military Awards in December 2011.
Here is the closeup:
It is widely believed that this (along with the matching bracelet) were wedding gifts.
The necklace is made of a connecting chain of small diamond circles running around the neckline. Suspended on each circle is a short straight line of white gold studded with diamonds. In between the circles are longer lines of diamonds. The lines alternate between short and long lengths to give it a unique pattern. At the very tip of each line is a flower-shaped ruby with a diamond center. I believe the whole necklace is set in white gold.
Given the design and weight of this necklace, I believe it has a fold-over, snap-shut clasp in the back. A necklace like this would need a sturdy clasp, and a lobster clasp would not seem strong enough to hold it in place.
My Small Obsessions received notice that this necklace and bracelet is worn by others, but in sapphires and not rubies. The photo I linked here shows the necklace and bracelet on Samira Said. I’m not sure if Kate’s was customized to have rubies (instead of sapphires), but I know that her necklace is not custom made in terms of design, because Samira owns the same necklace with different stones.