Polymorphism is one of the many fascinating properties of some minerals.  Polymorphs are minerals with the same chemical formula but different internal arrangement of  molecules.  This results in different external crystal shapes (called “habits”).    The resulting mineral depends mostly on the combination of temperature and pressure that were present at the time of formation.  A polymorph pair that may be familiar to you is diamond and graphite.  Both are composed of a single element, carbon (C), but crystallize very differently as shown below.

This example shows you how very different polymorphs can be, both in how they look and in their physical properties; graphite has a hardness of 1-2 on the Moh’s Scale while diamond is 10.  Another polymorphic gemstone that I use in  jewelry is kyanite (Al₂SiO₅): both andalusite and sillimanite are  polymorphs.

Marcasite and pyrite are also polymorphs with the chemical formula FeS₂  (iron sulfide).  Both have a hardness of 6-6 1/2, however marcasite crystallizes in the orthorhombic system while pyrite is isometric. In addition, marcasite crystals are not very stable so it turns out that marcasite jewelry is almost always made from pyrite.  I guess the name “marcasite” sells more jewelry than “pyrite”!

Natural pyrite cubes and assorted pyrite beads from my collection.

Pyrite is one of the best known minerals – almost every child has a piece in their rock collection.  Pyrite crystals can occur as near perfect cubes, or as pyritohedrons.  Pyrite is often called “fool’s gold” for it’s bright brassy color and metallic luster.  Next to real gold, the difference is obvious, but if you’re out mineral hunting and a piece of pyrite sparkles in the sun, it’s easy to be confused. As mentioned above, pyrite has a hardness of 6-6 1/2 while pure gold is only 2.5. Pyrite is an iron mineral so it is relatively heavy.

Lapis lazuli is a rock composed of three minerals, lazurite, pyrite, and calcite.   I enjoy using pyrite as an accent to bring out the pyrite included in lapis lazuli (and it looks great with azurite and other gems too).

Pyrite should not be cleaned with water; clean it with a soft cloth. Over time, pyrite may tarnish or dull and can leave black residue, so you may not want to wear it over light colored fabrics.

Now you won’t be a fool when choosing jewelry with pyrite!

References:
https://www.gemselect.com/other-info/marcasite-pyrite.php
https://www.mindat.org/min-2571.html
https://www.mindat.org/min-3314.html