A pseudomorph (which means false shape in Latin) is a mineral that has replaced another mineral's chemistry or structure with its own without changing the outward shape of the original mineral while color, hardness, and other properties change to those of the replacing mineral. Transformations from one mineral to another are not unusual in nature, but preserving the outward shape of the original mineral is! The end result is that the crystal appears to be one mineral but is actually another.
Crocidolite
Na2(Fe2+3Fe3+2)(Si8O22)(OH)2
Fibrous asbestiform Riebeckite (Amphibole)
Size: 10 cm
Tiger's Eye
SiO2
Quartz pseudomorphs after Crocidolite with preserved fibrous habit of crocidolite
Size: 2.5 cm (top, left), 10 cm (bottom)
Slab of Tiger Iron
Tiger Iron is a rock composed of tiger's eye, jasper and hematite as part of a Banded Iron Formation, a sedimentary sequence laid down some 2.5 billion years ago. The black parts of this polished slab are thin layers of hematite, and the red layers are jasper. The bright golden areas are part of the same layers, made of tiger-eye quartz. It forms when iron-bearing quartz replaces the fibrous mineral crocidolite.
Size: 55cm
Pyrite FeS2
Aggregate of equant cubic Pyrite crystals
Size: 10 cm
Limonite
FeO(OH) . nH2O
Limonite pseudomorphs after Pyrite with preserved cubic habit of Pyrite
Size: 8 cm
Stibnite
Sb2S3
Acicular Stibnite crystal and aggregate
Size: 9 cm
Stibiconite
Sb3O6(OH)
Stibiconite pseudomorphs after Stibnite with preserved acicular habit of Stibnite
Size: 8.5cm (rod), 13 cm