Kate wore this necklace to Wimbledon in 2012.
From the website:
“The necklace is made of solid sterling silver gold plated with 18ct yellow gold. The acorn is around 9x8mm.
This necklace comes in three different lengths – 35, 41 and 46cm but as with all KristinM jewellery this can be altered at your request.
Also available in white or rose gold.”
The pendant is a yellow gold plated acorn with a very detailed and sculpted image, made to closely resemble an actual acorn. It hangs from a long chain. I think the chain is about 20 inches or more because it falls lower on her chest than an 18 inch chain would.
Kate also wear a box chain with this necklace.
Some are speculating the chain could be from Links of London. I am not sure, but if it is I would think Kate would wear it more often. It may just be a silver chain owned since before her marriage.
Here is the necklace on Kate:
This necklace costs £140 or $200 USD.