What do the margins look like? I'm having a hard time using the picture you provided. If they are weathered, damaged, decomposed, partially digested, etc., it may be impossible to tell. But if they are sharp and appear undamaged, sponges. It appears to be too large to be from diatoms.

Thank you very much for your answer. In most cases the margins can be clearly distinguished from the rest of the sickle, as you can see on the attached picture. The samples went through steps of chemical organic destruction. Their appearance, however, was hardly affected. I also think that the sickles are too big to come from diatoms. If these are actually sponge residues, what would be the best way to dissolve them?