In one aspect of the invention, a method of forming polycrystalline diamond comprises providing a can assembly comprising a preform of diamond powder with a first catalyst, the preform being adjacent a solid substrate with a second catalyst. The pressure and/or temperature on the can assembly increases until the first catalyst and carbon from the diamond powder begin to form a melting composition within the preform. The pressure and temperature may be held substantially constant on the can assembly until the melting composition has catalyzed crystalline growth in at least a portion of the preform. The pressure and/or temperature are then increased on the assembly until the second catalyst within the substrate is released into the preform.