Abstract: Weighting material for a drilling fluid, e.g. a drilling mud, is produced by firing iron oxide solids, e.g. iron oxide ores, calcined pyritic ores or iron-oxide-containing products of other metallurgical processes, in a circulating fluidized bed or rotary furnace at a temperature of 1210.degree. C to 1400.degree. C in an oxidizing atmosphere in which the oxygen content is augmented to transform magnetitic components to oxides of lower magnetic susceptibility. The product is then cooled slowly at least until it reaches a temperature of 780.degree. C, also in an oxidizing atmosphere. The weighting materials have extremely low magnetic susceptibility and thus can be used without interfering with inductive monitoring of the position of the drill string.