OK, so shell shards are used for crafting, it says so.
Then I found "Straight Pin", basically throwing knives, and there was a counter saying I had 12 of them.
I assumed at some point I'd learn how to craft replacements. Since I didn't know how to replace them, I didn't use them at first.
A while after using one, I noticed I had 12 again.
What?
Turns out if you use them up, and sit down on a bench while having shell shards, you automatically replenish your supply.