I’ve thought about things like this for manufactured goods when green options aren’t available. If you want to market to the green-minded, you could put together a package in which you work in the price of carbon-offsets for a “greener” piece of jewelry. It might have to be a best guess, but if you know who’s doing the work, you might be able to get an idea of what the carbon costs are…

Obviously, this doesn’t mitigate the mining, but it might provide another way of offsetting energy use.