A single piece of glitter is a small reflective square. As it falls through a fluid (water or air) it will tumble. If there is a light source it will reflect that light source into your eye momentarily when it is at the appropriate angle.
My advice would be to set up a comp where you have a tiny square that rotates (X, Y, Z) in 3D. Then I’d create a light. I’d set the Specular Reflection of the solid way up and the Metal way down (so it’s very shiny). Then I’d place my light appropriately (head-on light or over head light).
I’d bring that pre-comp into my working comp, hide it, and set it as the particle (Random, Play Once).