The halo's main body is made of liquid flying birds with a metal texture similar to mercury. The can become different shapes

You can think of it as a state of pressure

Most common shape Flying birds halo (Left) Flying Hollow birds halo (Right)

With this picture you can understand it better

Sorry I can't draw metal textures

The birds fly in a line and eventually become a ring

It's only visible when being used It is hidden under normal light The halo can reflect the surrounding enviroment