I found a ring that is really similar, but the color is not as vibrant as the one that Cameron's wearing. The ring is available via net-a-porter.com and designed by Monica Vinader. It costs $250.
Here's the link! http://www.net-a-porter.com/product/30752
You can find a similar style on lolarose.co.uk
I have one similar from Tarina Tarantino. Not exactly posh jewellery, but I love it and gets loads of compliments when I wear it!