Best place nationwide to get jewelry - Costco. Seriously, all of their jewelry is stunning and priced right. If you want to spend 12k, there is a lot to choose from:
http://www.costco.com/jewelry.html
Chicago also has an area called Jewelers Row on Wabash where you can negotiate a really good deal on a ring if you are in the Chicago area. I had my wifes custom made there.
Some suggestions - dont buy odd cuts like heart shape or pear shape as they dont hold as much value as regular cuts. Princess (square) cuts are in right now and sell for a premium. Bring a magnifying loupe with you to inspect the diamond carefully, and never trust a jewelers appraisal.