A process of making a crystalline powder and the powder. The process includes providing at least a precursor material; hydrothermal synthesis to create a predetermined amount of boehmite as an intermediate product from the at least precursor material; hydrothermal synthesis to convert at least a portion of the boehmite to alpha alumina, wherein any remaining, un-converted boehmite is attached to alpha alumina. The process may be part of a process to make and extrudate and the extrudate. The composition and extrudate may include alpha alumina crystals with surface adhesions of boehmite. An apparatus for hydrothermal synthesis of high purity alpha alumina powder, the apparatus including an autoclave with titanium liners, a pressure relief system and a heat exchanger.