The shown pseudomorph specimen is displayed at the Loomis Museum with the following description:
This pseudomorph is a quirky combination of milky-colored quartz and dull-gray pyrite. It was found at the Sulphur Works and has the look of unshorn wool.
Quartz is an important rock-forming mineral that appears in different crystalline forms as well as colored varieties (for example, see The Quartz Page)—and in combination with other minerals. In a pseudomorph (literally meaning “false form”) a mineral replaces the structure and composition of another mineral with its own, but preserves the outward shape of the original mineral. The usual mineralogical terminology for a pseudomorph is “replacer after original.” Such a designation syntax, unfortunately, is not used in the description of the specimen above. Quirky!