An improved high pressure apparatus can include a plurality of complementary die segments. The die segments can have inner surfaces which are shaped to form a die chamber upon assembly of the die segments. A pair of anvils can be oriented such that an anvil is at each end of the die chamber. To prevent the die segments from being forced apart during movement of the anvils force members can be connected to the die segments. The force members can apply discrete forces to the die segments sufficient to retain the die segments in substantially fixed positions relative to each other during application of force by the pair of anvils. Using such a high pressure apparatus can achieve pressures as high as 10 GPa with improved useful die life and larger reaction volumes. Additionally, high pressure synthesis of various crystals such as diamond, cBN and the like can be carried out using temperature gradient methods wherein the reaction assembly is oriented substantially perpendicular to gravity during application of high pressure. Orienting the reaction assembly in this manner can avoid detrimental effects of gravity on the molten catalyst, e.g., convection, hence increasing available volumes for growing high quality crystals. Multiple reaction assemblies can be oriented in series or parallel, each reaction assembly having one or more growth cells suitable for growth of high quality crystals. The unique split die design of the present invention allows for particularly effective results and control of temperature and growth conditions for individual crystals.