Manufacturing of gemstone diamonds, as opposed to industrial ones, is only possible within the last few years, particularly if you require "white" instead of yellow. I know of only one company in the business, and even they can only go up to half of one karat so far.

www.apollodiamond.com

Another way to avoid supporting the African diamond warlords is to buy the ones that are mined in Canada, although I don't know what company to go to for that.