Abstract: Method and apparatus for producing chopped rock fibers, such as basalt. A furnace is preferably used to melt rocks, and a drawing assembly is used to draw the melted rocks into fibers. Prior to chopping the fibers to selected lengths, a preconditioning assembly operates to induce mechanical defects in the drawn fibers. Preferably, the preconditioning assembly comprises a shock cooling stage and/or a fiber twisting stage. The shock cooling stage significantly reduces the temperature of the drawn fibers, such as by the application of a stream of fluid such as water or steam to the drawn fibers. Additionally or alternatively, the shock cooling stage preferably comprises a liquid coolant bath through which the drawn fibers pass. The fiber twisting stage preferably utilizes a twisting assembly such as a roller to induce a torsional force upon the drawn fibers to continuously twist the fibers about an axis thereof.