The brooch looks like a peacock with the shape of a peacock's body and head at the bottom center and with nine light sea-green tail feathers coming out from it.
It is currently unknown what it looks like in its disguise mode.
The powers granted by the Peacock Miraculous are unknown. However, as seen in "The Collector" on a digital version of the tool's page in the Miraculous spellbook, the tool appears to be a single feather or something similar. The Peacock Miraculous allows it's user to create allies for the Moth Miraculous' akumatized villains or heroes. Specifically how this is achieved is unknown.