Pinstem

A pinstem is a jewelry finding affixed to the reverse of a broochpin or tie tack specifically designed to pierce fabric for the purpose of fastening the object to an article of clothing. The hinged pinstem on a brooch is usually held by a clasp or catch, ranging in design from a simple “C” to one with an intricate safety mechanism.
Reverse View of Antique Amethyst and Garnet Spider Brooch Featuring the Pinstem.

Reverse View of Antique Amethyst and Garnet Spider Brooch Featuring the Pinstem.