Supply and demand is more of a guideline than a strict rule (as is almost everything in economics). Monopoly pricing could result in lower than market pricing

Supply and demand is more of a guideline than a strict rule (as is almost everything in economics). Monopoly pricing could result in lower than market pricing if 1) they have enough supply to satiate the market demand and 2) they want to continue to price any competition out of the market. By keeping prices lower they stop existing competition from being able to make a profit and they stop new competition from being able to justify the costs to start mining operations. In other words, in a market that anyone can enter, a monopoly can establish a continued monopoly by ensuring that they are the sole supplier of goods in that market. If it becomes too profitable, then other competition will naturally start eating at your market share. Note: I'm not saying this is what happened with DeBeers.