What reaction could rubidium undergo in a vacuumed chamber with Macor ceramic and glass?
One candidate for reaction with the rubidium is silica, $\ce{SiO2}$.  Reference [1] reports that black powders are formed when alkali metals are contacted with silica.  Such powders are more stable in dry air than ordinary alkali metals.  The reference suggests that electrons are imparted from the alkali metals into the silica forming an electride complex.

Reference


Shatnawi, M.G., Paglia, Gianluca, Dye, James, Cram, Kevin, Lefenfeld, Michael, Billinge, Simon, "Structures of Alkali Metals in Silica Gel Nanopores: New Materials for Chemical Reductions and Hydrogen Production", Journal of the American Chemical Society 129, 1386-92 (2007).  DOI 10.1021/ja067140e