Highly aggregated color centers in wide-band-gap dielectric calcium fluoride crystal, heated in a reduction atmosphere of Ca vapors (an "additive coloration" of the crystal), have been studied using spectroscopic and atomic-force microscopic techniques. These centers are large aggregates of anion vacancies and electrons that transform into metal structures. Evolution of these structures with increasing the number of such components has been traced from individual two-dimensional particles to large metal film fragments. (C) 2012 Elsevier B. V. All rights reserved.