Longevity of freshwater pearls.
I think it is the duke of Norfolk that still has a FW strand from the first Queen Mary. It is from the river Tay in Scotland. It is in perfect condition after 400 years. It is in one of those traveling shows and we have put up pictures of it here before.
In general, I do not think a distinction was made between sea water and freshwater pearls in the heyday of natural pearls, both were used and reused over the decades, even centuries, and most of them in the Book of the Pearl by Kunz (1908) are still being restrung and worn. That book has literally hundreds of people wearing piles and piles of pearls. By the 19th century, it looked like all the royality and nobility were saturated with pearls and wore them lavishly for photographs. That book is hard to find and expensive, even though it is a Dover book, but the entire book is online- I am sure there is a link in the Pearl Book and Resources section.
I think the black dye in CFWP was not as good ten years ago, (and neither was the quality of the pearls) but I only have those rice krispie looking pearls from that far back- none of which I have worn every day, none of which are black either. They are shiny metallic colors, bronze, gold, silver.
I do have some white pearls from Pattye that have chips missing from the surface. The nacre showing through the chips is just as shiny as the parts that aren't chipped.
So even though CFWP of good-to-great quality are still youing, a look at history tells us something about the longevity of solid nacre pearls.