Abstract: Method and apparatus for compaction of powder of high density boron nitride, having an initial particle diameter of at least two microns, into a high density crystal aggregate of between 1 mm up to about 1 cm in diameter, the resulting crystal aggregate having a zincblende form, the wurtzite form or mixtures of the zincblende and wurtzite forms. High density aggregates of boron nitride, preferably having particle sizes with diameters at least 2 .mu.m, are placed in a hollow, substantially cylindrical first container of metal, and the first container is surrounded by a fluid-like material of metal or metal powder having a shock wave velocity V. The first container and fluid-like material are placed in a substantially cylindrical second container that is purged of substantially all air and sealed. The second container has a rigid side wall and two rigid end walls.