Gold mining is a nasty thing. Often modern gold mining involves open pit mines, which destroy all life above the area being mined. Or, they use a cyanide leaching process and for each small amount of gold tons of waste is left behind. Entire eco-systems have been destroyed by gold mining, for examples see this article. You can also check out the No Dirty Gold website for a lot of interesting information. With a little quick research you can find many other examples from many sources. Here's a little bit of additional information about gold mining you can use as a starting point. Diamond mining is not very good for the world either, in terms of labor costs or the environment.
So if you want to wear or buy gold or diamonds, what can you do? As I am getting married, want a wedding band, and don't want to contribute to gold mining, I looked into various options. My favorite choice is to buy from an antique shop or a pawn shop, as buying an existing ring does no further damage to the environment. While it can be time consuming to find a ring you like this way, as you have to find one your size, the internet makes it easier. By far the best site I found (and the one where we ended up getting both rings from) is The Marlene Harris Collection. It's easy to search there, they have a great selection, and if you tell them what you are looking for they will keep an eye out as new inventory comes in. Their service is good, and you can buy a ring and return it if you don't like it, so you can "try them on" by mail.
There are alot of other options out there too. Here are the ones I was able to find:
Sumiche Jewelry Co.
Jewelry by Da'oud
Decades Vintage Jewelry
Leber Jeweler Inc.
Antique Jewelry Exchange
Antique Jewelry Mall
And of course, there's always Ebay, and your local pawn shops. You can also see the Green Guide to Jewelry for additional info.
Personally, I hate diamonds and will never own one. I was looking for a simple yet unique and classy gold band, and luckily after a few months of looking I found a great one. It's a civil war era ring, and the men's ring we ended up getting is from the same era and also totally unique. I highly recommend "going green" when it comes to jewelry.