POPULAR PSEUDOMORPHS - More Common Varieties Enhance Any Collection

In part one of this two-part series, which appeared in the December 2020 issue of Rock & Gem, we explained pseudomorphs as minerals whose normal form has changed, so they may look like the original but are now another mineral.

- BOB JONES 

That change results in pseudomorphs, sometimes called pseudos, including replacements, casts, paramorphs and others. Some of these pseudomorphs are very attractive and common enough to add to any collection.

This article will describe some of the common and attractive pseudomorphs that are most likely available to collect. These examples include minerals and lapidary materials like petrified wood, as described in the first part. The most common lapidary material that can contain pseudomorphs are agates, especially banded agates. We may not think of agates as having pseudos, but many do.

Banded agates form in gas vesicles or openings in volcanic rock. One theory says silica-rich waters invade these open pockets and deposit the silica as alternating bands of color. Another theory suggests silica gel in the still fluid volcanic rock accumulates slowly to form rounded masses that cool, forming agate banding. In either case, the solutions in the cavity also contain molecules of other minerals like carbonates and zeolites, which tend to crystallize first on the walls of the hardening volcanic rock. The formation of the agate bands follows, setting the stage for pseudomorphs to develop.

DYNAMIC DEVELOPMENT