Aluminosilicate gel-derived glasses of 0.5CoO . 5Al(2)O(3). 94.5SiO(2) composition were obtained by the sol-gel method. The samples were transparent, and their color depended on heat-treatment temperature. The gel sample presented a pink color but, when heat-treated at temperatures higher than 43 degrees C, acquired amethyst and purple colors. The samples heat-treated at 120 and 250 degrees C showed an evident modification of color from amethyst to pink when in contact with air humidity. The changes from pink to amethyst and purple were related to the ability of Co(II) to act as a network former and/or modifier. Ultraviolet (UV), visible (VIS) and near-infrared (NIR) spectrophotometry, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were the techniques used to obtain information about the relationship between the glass structure and the color of the samples, (C) 2000 Elsevier Science Ltd.