Humidity, pyrite and salts
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Pyritized specimens are the known problem
Pyrite in a fossil can oxidize in humid conditions, producing sulfates and acid that expand and crack the specimen. Ammonites and some shales are commonly affected, and the damage is progressive rather than one-off.
It looks like a white or yellow bloom
Powdery efflorescence on the surface, sometimes with a sulfurous smell, and cracking that follows. By the time a bloom is visible the process has been running for a while.
The response is dry air
A sealed container with a desiccant, kept stable and cool, is the standard conservation answer because it removes the water the reaction needs. That is exactly what this shelf plus a sachet provides.
Calcareous material has its own version
Shell and other carbonate material can deteriorate in the presence of acidic vapors, which is one reason the storage material around a specimen matters as much as the jar. Avoid anything that off-gases.
Salts from marine or desert matrix
Soluble salts in the matrix can migrate and crystallize as humidity cycles, pushing surfaces apart. Stability matters more than any single humidity figure, so a sealed jar in a cupboard beats an open shelf in a bathroom.
The desiccant is a consumable
It has a capacity, it absorbs from the moment it is exposed, and a saturated sachet is doing nothing. Replacing it on a schedule is the practice; installing it once is a false sense of security.
Inspect twice a year and write the date down
Looking for bloom, cracking and any change in the matrix. Every failure here is slow, and a schedule is the only thing that catches a slow failure while it can still be acted on.
Not every fossil needs this
A well-mineralized tooth or a silicified invertebrate in a stable room is not under chemical attack, and treating everything as fragile wastes effort that the genuinely unstable specimens need. Knowing which of yours are pyritized is the useful work.