A stemming plug (60) for stemming a blast hole in a mine is disclosed. The plug has first and second wedge-shaped members (62′, 62) manufactured from a suitable plastics material. The first wedge-shaped member (62′) has a first sloping face received in sliding relationship with a matching face of the second wedge-shaped member (62) wherein, in use, when the first wedge-shaped member (62′) is positioned nearest to an explosive material in the blast hole it has a larger surface area facing the explosive material than the second wedge-shaped member (62). In use, when a Shockwave from initiation of the explosive material in the blast hole encounters the first wedge-shaped member (62′) it acts as a piston, sliding on the second wedge-shaped member (62) so that both wedge-shaped members exert diametrically opposed forces against the wall of the blast hole to lock the plug (60) in place. The two wedge-shaped members (62′, 62) may be substantially identical, thus significantly simplifying the manufacturing process.