Another possibility is that I'm reversing cause and effect here. Perhaps demand started soaring first, which inspired the entrance of the new player, who quickly shored up the gap in demand -- thereby restoring supply/demand equilibrium to the market, and keeping prices stable. Not out of the realm of possibility.

In such a scenario, De Beers wouldn't necessarily or aggressively chase the second player into the new market. It's doing just fine in its existing markets, and it doesn't want to risk setting off a price war over the new market.