An Epimorph occurs when one mineral forms on another one, and then the first mineral is dissolved by acidic liquid or gas, leaving a cast of the original mineral in the latter one. 

Epimorph from Turkey of Quartz

Here is an epimorph from Turkey of Quartz (var. Smoky and Amethyst) that formed on another mineral - probably Calcite. Calcite is softer than Quartz, or lower on the Moh’s scale of hardness, therefore it is dissolved more easily. Because of this, the Quartz was able to remain intact while the Calcite was dissolved away, leaving this beautiful cast on the bottom of this piece. 

In this epimorph from Colorado, the white Quartz has formed on scalenohedral (aka “dogtooth”) Calcite, and the Quartz was left intact as a perfect cast of the spiky, cone-like Calcite crystals.

A pseudomorph is when a mineral replaces something else, keeping the shape of the initial substance rather than its own. Petrified wood is a great example of a pseudomorph, in which the organic material of the wood has been gradually replaced by silica until the entire branch or log is stone, in the exact shape of the wood. It can even retain the grain of the wood and the bark that was on the tree.

Petrified Wood (Pseudomorph)

Many fossils are pseudomorphs - Ammonites are another example, as the organic material of these creatures has been replaced over time by Silica, Calcite, Pyrite et cetera.

Many fossils are pseudomorphs - like this Ammonite

Another way that pseudomorphs occur is when one mineral replaces another mineral. Here is a pseudomorph from Pennsylvania of Goethite after Pyrite, or Goethite that has completely replaced Pyrite and kept the shape of the original mineral now that it is gone. The epimorphs that we first looked at are also considered pseudomorphs, they are called ‘incrustation pseudomorphs’ because they assumed their ship by encrusting the initial mineral rather than replacing it. 

Pseudomorph from Pennsylvania of Goethite

As you can see, minerals can be sneaky, shape-shifting bastards. But don’t worry, it’s just one more reason to love them. They are complicated, interesting individuals and the more I learn the more I realize there is to learn about them! I love to share what I learn and I hope you enjoy it too.