According to the verse above wherein G-d commanded Moses to make the laver, it must be constructed of copper. Although many other Temple vessels were constructed of costlier metals such as gold or silver, thus giving greater honor to the will of G-d - the laver was to be made exclusively from copper. Even later, throughout all the subsequent generations of the Holy Temple, and even when the nation knew times of great prosperity and decorated the entire Second Temple in gold, the laver still remained of copper.
The Midrash (Bamidbar Rabbah 9:14) relates that the original laver was made from the contributions of the righteous women of Israel, who donated their shiny mirrors towards this cause. These mirrors, made of highly polished copper, were melted down and it was from these that the laver was created. This act of sacrifice was precious in the eyes of the Holy One - the fact that the women cared more about fulfilling the word of G-d than about their own appearance. He declared that the laver must be of copper throughout the ages, to invoke the merit of these righteous women, so the memory of their action will always be before Him.