Not a thing wrong with Biwa! They're a special pearl, cultivated in a lake in Japan called Lake Biwa, which eventually became so polluted that the molluscs that produced the pearls couldn't thrive. Because of that, and a big die off, the lake isnt producing anymore, so the Biwa pearls that exist are valued.
However, the Chinese took the mollusc that produced this kind of pearl and started cultivating it in freshwater lakes in China, and they produce pearls, soooo...
If you've got provenance (proof that the pearls actually came from Lake Biwa), you've got a niche market. Without that provenance, you've got a pretty freshwater pearl. Nothing wrong with that, either!
Another thing we're dancing around is whether your pearls could be natural, meaning they were grown without any help from humans, ie not cultivated. Natural pearls are rare, and expensive, sometimes very expensive and have a sort of reverence associated with them in some circles.
I'm going to suggest you go over to
www.kojimapearls.com. Sarah has a blog and she recently posted about a cache of Biwa pearls that were unearthed in a pearling family vault. I'm going over there to look at those Biwas again.
Or, we could go up to the search button at the top of the page and search
Biwa and see if Sarah's recent post pops up fast...