What to print a label on
Updated
An unusual requirement: the label has to survive an environment that some specimens actively make hostile.
The label has to outlive the specimen
A decaying pyrite specimen produces sulfuric acid, and that acid attacks the label and the box alongside the mineral. The paper frequently fails first.
Acetic acid vapor from a wooden cabinet or a degrading plastic does the same more slowly to anything made of cellulose.
So label stock is a conservation decision, not a stationery one.
Paper
Acid-free and lignin-free, ideally buffered. Ordinary printer paper yellows and embrittles within a decade in a poor environment.
Heavier stock survives handling. A label is picked up, turned over and put back hundreds of times over a collection's life.
For anything known to be reactive, keep the label physically separated from the specimen inside the box, or in an outer sleeve, so the acid has further to travel.
Ink
Pigment-based and lightfast. Dye-based inkjet ink fades and runs if it ever gets damp.
Laser toner is fused rather than absorbed and survives well.
Pencil is genuinely excellent for field labels: it does not run, does not fade and is legible on paper that has been wet. The old collections that are still readable are frequently in pencil.
What fails: ballpoint over time, felt tip quickly, and anything water-soluble the first time a box gets damp.
Where it sits in the box
Visible without moving the specimen, so nobody has to handle the stone to read it.
Not underneath, where lifting the specimen to read the label is how specimens get dropped.
And tucked rather than glued. An adhesive label in a specimen box becomes a conservation problem of its own once the adhesive ages.