I would guess that the natural diamonds' prices have fallen as the synthetics increasingly set a price floor. They have such large stockpiles of diamonds and they have to bring in revenue, they can't simply pretend the synthetics don't exist on the market and price accordingly. Or to put it another way: for such perfect substitutes and commodities, the prices should be near-identical, and so it is wrong to try to imply that consumers haven't saved a ton of money because the prices are similar; you need a time-series of prices.