Shells, sea glass and sand
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Seashells
The defining beach find. The legal questions are occupancy, species and jurisdiction; the storage question is salt, which stays in the shell unless it is rinsed out properly.
Salt is hygroscopic
It draws moisture from the air, which means an unrinsed shell in a sealed jar creates its own damp microclimate. For a shell that matters little and for anything metal in the same jar it matters a great deal.
Sea glass
Chemically stable, physically durable and the easiest find in this property to store. Clear containers suit it because the whole point is the colour.
Though it is still beach material
And some beaches regulate removal of anything. Sea glass is a bottle that has been in the sea, which in some jurisdictions makes it litter and in others makes it an artifact.
Beach sand
Its own leaf and its own archetype, a 5 to 120 mL lab sample format. A labelled sand collection is a genuine thing people keep, and it needs a small sealed container and a reliable label far more than it needs anything else.
And sand is widely restricted
Quantity limits, park prohibitions and in some countries outright bans with fines. It is the find most likely to be assumed harmless and most likely to be regulated.
Dry everything before sealing
Completely. A damp shell or damp sand sealed in a jar grows mould, smells, and in the case of sand clumps permanently.
The container shelf suits these well
298 containers from 5 to 500 mL, 94 of them clear, with 101 PET single wall jars. For shells and sea glass that is the right shape and the right size range.