It's true. The Chinese 12k is one of my favourites. The charnley forest when it was still quarried it was very very cheap, that goes also for the escher. It's stock market. When the mine closes, if the stone has a reputation, the demand rises but there aren't new stones to cover the market's needs, and the price rises for the already circulating ones. This is not the only factor, just an example. An expensive stone is not a better stone. And every cheap natural stone, the next 30 or 50 years will become very very expensive, so, collect as many as you can! (few of the Jnat mines are still operating, but the Japanese natural stones have become a legend, that's another factor that changes the price)