I shop most of my fine jewelry on bloomingdales.com and tiffany.com. Those jewelry I will keep them in my collection almost forever.
But if I just want some costume jewelry that can match my outfit + in season, I go to nastygal or misschopin. Their accessories are very affordable and matches well with outfit. The downside is that they offer fashion jewelry, so some items might not be as timeless as my other tiffany pieces.
I'm in the U.S. but got interested in Monica Vinader jewelry after Kate Middleton recently wore some of their designs. Check out this article: "Fast-growing Monica Vinader has successfully tapped a gap between fine and fashion jewellery. Now, the company is moving ahead with its long-held plans for global expansion." http://www.businessoffashion.com/2014/11/fine-fashion-jewellery-monica-vinader-poised-expansion.html
I recently started to purchase on Etsy, too. I LOOOVE it! I bought three pieces of jewelry from Etsy so far from two different shops, and have not been disappointed