Padding and separation
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The problem
These finds are hard, irregular and often fragile at an edge. Loose in a container they move, and movement against glass or against each other chips and abrades.
Which the container cannot solve alone
A jar is a jar. The separation has to come from what goes in with the object, and that material is then in a sealed atmosphere with it.
So choose inert padding
The same logic as the numismatic property: polyester, polyethylene and polypropylene are the plastics conservation guidance names as suitable, and ordinary tissue and cotton wool are not inert in the way people assume.
Cotton wool snags
On anything with a textured or crystalline surface, which includes most shells and many shards. Fibres left behind are a cleaning job later.
One object per container is the clean answer
For anything individually valuable. It removes the padding question, the separation question and the labelling question in one move, and the shelf runs down to 5 mL for exactly this.
Bulk is a different job
A jar of sea glass is meant to be a jar of sea glass. The pieces are already tumbled and the abrasion that worries a collector of shards is what made the sea glass in the first place.
Fill level matters
A part-filled jar lets contents move in transit. Either fill it or pad it, because a half-empty jar carried to a show is the condition that does the damage.
Foam that ages is worse than no foam
Some foams break down into a sticky residue over years, which is a conservation problem rather than a tidiness one. Inert plastics and plain polyethylene foam are the safer choices.
Think about the trip, not the shelf
Most damage happens in transit to a show or a move, not on a shelf. Packing for the worst journey the collection will take is what sizes the padding decision.