From Wikipedia: Exact formulas for this compound varied by manufacturer and over the course of time, but it was typically composed of about one-third shellac and about two-thirds mineral filler, which meant finely pulverized rock, usually slate and limestone, with an admixture of cotton fibers to add tensile strength and carbon black for color (without this, it tended to be a "dirty" gray or brown color that most record companies considered unattractive).
Others claim the main filler used in 78s was carborundum, aka silicon carbide an extremely hard industrial abrasive.Whatever the filler is it grinds away steel needles pretty quick. RCA recommended changing to a new needle after playing two sides.