Word Wednesday: Pseudomorph

Welcome to Word Wednesday, Wolfstone Rock Shop’s weekly look at the language of rocks, minerals, and gems.

Pseudomorph means “false form.” It describes a mineral specimen whose present composition differs from the mineral that originally created its outward shape.

A geological costume change

Imagine one mineral forming a well-shaped crystal. Later, groundwater or another geological process alters or replaces that mineral while preserving the original crystal outline. The result looks like the first mineral on the outside but is made partly or entirely of something else.

How can it happen?

Pseudomorphs can form through replacement, alteration, encrustation, or loss of a component. Limonite after pyrite and malachite after azurite are well-known examples. Some specimens preserve remarkably fine faces or edges even though the original material is gone.

Why collectors care

A pseudomorph records a sequence of events, not merely a mineral name. A good label should state the current mineral “after” the original one—for example, goethite after pyrite—and preserve reliable locality information.

See this texture and other mineral features in person at Wolfstone Rock Shop, 1857 N Casaloma Dr, Appleton, WI 54913. Inventory changes often, so call (920) 809-8577 if you are looking for a particular example.