what's the stuff called that is used in the deep south on flatdeck reroofs, small
pebbles that look like grey and white stones, attached to surface of the topcoat and
also thrown onto the melted tar ?

while the tar was still liquid the crew throws a little of this sand like substance
over the top coating.

does it serve a cosmetic purpose or is it functional in reducing sun exposure to the
surface?

if rain washes away some of this from the surface, is it worth reapplying?