I'm hardly an expert, but I think about that topic a lot. It's very painful to me to think that someone or the Earth might have suffered and been exploited so I can hold a pretty crystal, even if I do use it to heal myself and others. I think crystals do take on the negative imprint of how they were mined, absolutely - but beyond that, even if they don't, my own mind won't tolerate it. If the standards of safety and pay for workers aren't at least adequate, and if the crystals are mined destroying the landscape, then I'm happier without them.
It's difficult to know everything about our crystals, however. I stay clear of stones whose provenance I know is dodgy, like Burmese stones, and as much as I can, buy crystals from sources that are reputedly clean. I've also found a few websites with information about ethically mined crystals and how to source them. And I am sensible - when I receive a gift, I don't look it in the mouth and question the giver.
Here are two:
http://www.kacha-stones.com/
http://www.alternativesnottingham.co.uk/intro.htm
These threads have information and interesting discussion:
http://www.tarotforum.net/showthread.php?t=9584&highlight=ethical
http://www.tarotforum.net/showthread.php?t=56144&highlight=ethical
http://www.tarotforum.net/showthread.php?t=97675&highlight=ethical
But to be honest, the information is scarce, compared to what we have on diamonds and other precious stones. It's a subject that's been ignored for the most part by the New Age crystal healing community, and it tends to bring up all sorts of embarrassed answers when questions are asked. Even on this site, some people don't think about it at all (and some maybe don't give a damn, I don't know).
So it's really a good thing when people do ask these questions - including in crystal shops, and push shop owners, practitioners and crystal lovers to be a more thoughtful about where they source their crystals! I don't think it's possible to heal ourselves and the Earth - at the cost of others and the Earth!