Menachos, 29a: It hints at the image of the Menorah, which came down from Heaven - a place of Taharah - (to teach Moshe how the Menorah looked - Torah Temiah).
2
See 25:31. This is in contrast to the Aron, the Shulchan and the Mizbe'ach ha'Zahav, which were made of wood and which were only overlaid with pure gold.