Clarity, and what the jar is for
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Why clarity leads
A fossil is bought and kept to be looked at. A clear jar is a cheap, sealed, stackable display case with a flat label panel, and 66 of the 139 containers on this shelf are clear.
PET dominates, and that is reasonable here
112 PET against 27 glass. PET is lighter, does not break when a tray is knocked, and for a dry mineralized specimen the chemical inertness advantage of glass matters far less than it does for a fluid-preserved one.
Where glass still wins
For a long-held reference collection, for anything where you want the best barrier against humidity, and for presentation. 27 containers, mostly straight-sided jars and mason and paragon forms.
Wide mouths are not optional
A solid specimen has to go in and come out without scraping the wall. The dominant necks here are 70-400 at 38 containers, 89-400 at 34 and 58-400 at 29, which is genuinely wide across the shelf.
Size by specimen, not by set
The range runs 50 to 500 mL. A trilobite and a megalodon tooth are very different objects, and matching the jar to the specimen looks better than a uniform jar with one item rattling in it.
A flat panel for the label
Which is more important here than in most categories, because the label carries the provenance and the provenance is the specimen’s value. The 3 square and rectangular containers on this shelf exist for exactly that.
And the label goes inside as well
A card in the jar survives a move that a stuck-on label does not. For a collection that will outlast its owner, the internal record is the one that matters.