Why are conflict minerals bad to purchase?

Conflict minerals (some example of exotic stones such as diamonds, rubies, emeralds, etc.) have a criminal economy behind them sometimes. In the past, people have come to harm due to these stones because of their price. Because such stones are expensive many people would like to get a certain profit out of them. Due to this many criminals have been attracted to controlling areas where such stones are being dug up - and with the purchasing of such minerals, this industry is being allowed to exist where people are being exploited.