What a pseudomorph is
A pseudomorph is a mineral that has taken on the crystal form of a different mineral, through replacement, alteration or infilling of a cavity left by dissolution.
The classic example is limonite after pyrite: perfect pyrite cubes composed entirely of iron oxides.
Reading them
The convention is to write the new mineral, then after, then the original: goethite after pyrite, quartz after fluorite, malachite after azurite.
They are more informative than ordinary specimens, because they record a sequence of events rather than a single moment.
The identification trap
A cubic outline suggests a cubic mineral, and pseudomorphs break that inference cleanly. Test hardness and streak rather than trusting the shape.
This is precisely why habit is treated as a hint rather than evidence.
Why collectors pay for them
A pseudomorph records a change in conditions — a fluid arriving, an oxidation front passing through, a cavity opening and refilling. That makes it evidence rather than merely an object.
Well-documented examples from a known locality are consistently more sought after than ordinary specimens of either mineral involved, because the sequence they preserve cannot be reconstructed from anything else.