A high temperature high pressure apparatus which comprises: an annular die having a straight cylindrical bore and a substantially conical face in adjacency outwards with each end thereof, a pair of tapered punches which are in opposed and axial alignment with the die so that a conical face of each punch is substantially in parallel with that of the die, a pair of inner gaskets, each of which is made of fired refractory and is arranged in direct abutment on the conical face of the punch and the bore of the die, a pair of outer gaskets, which are made of material of intermediate hardness level and is arranged in adjacency outside the inner gasket, and a pair of stopper rings of readily deformable but highly tough material and arranged in adjacency outwards to the outer gaskets.