A method for making a thermally stable cutting element may include forming an acid mixture containing two different acid species by combining an acid solution and at least one acid-forming compound, wherein the at least one acid-forming compound is provided in solid form, and wherein the at least one acid-forming compound produces an acid that is different than the acid solution; treating at least a portion of a sintered diamond body by placing the sintered diamond body in the acid mixture, wherein the sintered diamond body comprises: a matrix phase of bonded-together diamond grains; a plurality of interstitial regions dispersed within the matrix phase; and a metal material disposed within a plurality of the interstitial regions; wherein the treating removes the metal material from at least a portion of the plurality of interstitial regions; and removing the sintered diamond body from the acid mixture after a predetermined length of time, wherein at least a portion of the diamond body removed from the acid mixture is substantially free of the metal material and is a thermally stable diamond body.