If we want to identify a mineral without special aids, it is best done by observing certain morphological (shape of crystals and aggregates) and physical properties (colour, scratch, light, fission, fracture, and hardness). However, there are “tricky” minerals that do not appear in their own form but in the form of another previously formed mineral. These are called “false forms” or pseudomorphs. For example, goethite, which is an iron oxyhydroxide (FeOOH), often takes the form of various iron sulfides (pyrite, marcasite).
In the picture: Goethite pseudomorph after pyrite, Moneasa, Romania. Size: 2 cm. Photo by László Tóth