Almost all jewelry quality sapphires are man-made, and the cabochon crown sapphire chemically is the same as the clear sapphire see-through crystal of high end watches. Sapphire is also chemically equivalent to ruby (as most jewelry rubies are also man-made). The difference in red and blue is the impurities that are added (or occur naturally) to cast the red or blue color. There is nothing special about the Cartier cabochon sapphire except as a signature marketing tool for Cartier. It certainly does not cost a lot to make a more exacting color match. The replica makers could probably do it by adding 50 bucks to the price of the watch to get better quality wholesale sapphires (which may seem prohibitive for them when it comes to the effect on watch sales). But getting a perfect replacement might cost you far more than it's worth.
Someone should just sell the matching Santos 100 XL crown with higher quality stones separately. I bet it would sell well on the replica market.
