Right now, Petrified seashells are thrash-tier for mot player. I personally use q40+ seashells on my main (in large part because there's so much other stuff to do than producing curios to fill my study entirely) but q10 petrified seashells are something that you can barely throw at alts that wouldn't otherwise study anything.
My suggestion is not to outright buff petrified seashells, but add following crafts:
Craft 10 current petrified seashells into uncommon seashell that is better than petrified seashell
Craft 10 uncommon petrified seashells into rare petrified seashell that is better than uncommon seashell