A solid state seed crystal process for bulk conversion of a polycrystalline ceramic body to a single crystal body (of the same chemical composition) having the same crystal orientation as the seed crystal. The process comprises heating said body to form a monolithic join between the body and the seed crystal, heating the joined structure to reduce grain growth inhibitors and further heating the joined structure above the minimum temperature required for crystallite growth of the crystalline material, but not hot enough to melt and distort the original shape of the polycrystalline ceramic body during its conversion to a single crystal. This process has been used to convert polycrystalline alumina (PCA) bodies to sapphire having the same crystal orientation as the seed crystal by heating the PCA body, monolithically joined to a sapphire seed crystal, at a temperature above 1700.degree. C. without melting the body.