I've noticed too that it's easier to scratch freshwater pearls compared to the south seas and tahitians.
Just don't know the hardness range of each pearl based on source specie.
Anyhow, checked below sites. Pearl hardness is said to be around 2.4-4.5.
http://www.gemsociety.org/info/gems/Pearl1.htm http://gemologyonline.com/pearls.html
2.4 vs. 4.5 is a significant difference.
