Retroreflective Glass Beads
This sugar-like powder actually consists of high purity, high precision, 98+% spherical glass beads ranging in size from 180-250µm, with low air inclusion for excellent optical properties.
But why? Well! with the correct refractive index, these charactoristics are a recipe for retro-reflection, which is a term for a condition of light being reflected back towards its source - unlike a mirror, which will reflect at an incident angle, these beads always reflect light directly back towards where it came from, like cat's eyes at night. Commonly seen for painting stripes on roads, safety signage, personal safety gear for increased visibility, and plenty of other uses.
Optical properties aside, the spherical properties of this powder make it extremely slippery - you can't make a tall "pile" of it because it's so self-flattening, and it self compacts very well making it useful for estimating cavity volumes, for example if you're casting small precision parts in resin or epoxy and don't want to waste any guessing your mold volume. For reference, it's about 11cm³ per 29g/1 oz.
Also, if you spill any, it makes your work surface frictionless because of all the tiny spheres to roll on. We found this out by mistake.
When using this powder for it's reflectivity, there's one important thing to remember: In order to relfect as intended, they have to be in direct contact with the light. This means that mixing them into paint will not work as well, since many fo the beads will be submerged, but applying them to the surface of partially-dried tacky paint will work very well. Similarly, they work well applied to the surface of hot-melt adhesives or paint mediums. This is how many road stripes are applied!