Abstract: A method for producing a nanodiamond (n-diamond, p-diamond, i-carbon) in which a nanodiamond is removed from an activated carbon containing the nanodiamond. The activated carbon is prepared by carbonizing and/or activating a carbonaceous feedstock while restricting the presence of oxygen sufficiently to result in the formation of nanodiamonds embedded in carbon. The nanodiamonds can be separated and purified from the activated carbon, and can be concentrated by treatment of the activated carbon with an oxidizing agent. Also provided is a method for producing a nanodiamond, and particularly a nanodiamond fiber, by mixing a carbon source, a metal and an acid under conditions which result in nanodiamond formation. Nanodiamond fibers up to 2000 nanometers or more can be produced. The nanodiamond fibers can be woven or used to provide structural reinforcement for various materials.