We purchased a lab grown/man-made diamond on purpose. Even "non-conflict" diamonds are mined in immoral ways. And while in theory it is great to bring outside money into some of the poorest areas of the world, the work conditions and atrocities cannot be ignored[0][1][2].
Ideally people should move beyond diamonds entirely. There's plenty of precious stones. But in the meantime lab grown diamonds are the most moral way to buy a diamond.
[0] http://www.nbcnews.com/id/15842524/ns/world_news/t/diamonds-...
[1] https://en.wikipedia.org/wiki/Child_labour_in_the_diamond_in...
[2] https://www.huffpost.com/entry/all-diamonds-out-of-afric_b_2...