Should you reuse tumbling grit, and does it lose effectiveness

Tumbling grit genuinely does break down with use, coarse silicon carbide grit in particular wears into smaller, less aggressive particles as it grinds against the stones, which is part of the intended process, coarse grit is expected to progressively refine as a stage runs. Reusing already broken down grit for a brand new coarse stage on rough, unshaped material will simply take much longer to achieve real shaping since the grit is no longer as aggressive as fresh material. Most experienced tumblers use fresh grit for the coarse shaping stage where aggressive cutting matters most, and are more willing to stretch finer grit stages a bit longer if it is not brand new, since those stages rely less on raw cutting power.