A pin or clasp used to in relation to clothing.
Broadly speaking there were three main types of fibulae.
This is the "safety pin" shape and all of its variations.
This is the variety most like the modern "brooch".
The ring and pin configuration remained popular in the post-Roman period. The ring might be solid ot it might have a gap.
Although fibulae are often illustrated in the "pin downwards" orientation, this might not have been the norm for fibulae.