Russian diamond miner Alrosa PJSC joined rival De Beers in cutting prices for its gems, but with a twist — the company changed its billing system so buyers can’t tell which stones are discounted, or by how much.

The two biggest producers for months refused to cut prices even as sales collapsed during the coronavirus pandemic, to avoid giving up too much ground before demand returned. De Beers and Alrosa don’t publish prices for their rough stones and their dominance in the often-secretive diamond industry has allowed the two companies to dictate terms to their customers over the years.