Glass; but bronze mirrors were used in the OT period in Palestine.
Citation for mirror
Citation styles are based on the Chicago Manual of Style, 15th Ed., and the MLA Style Manual, 2nd Ed..
MLA
"mirror." In A Dictionary of the Bible. Ed. W. R. F. Browning. Oxford Biblical Studies Online. May 19, 2013. <http://www.oxfordbiblicalcstudies.com/article/opr/t94/e1268>.
Chicago
"mirror." In A Dictionary of the Bible. , edited by W. R. F. Browning. Oxford Biblical Studies Online, http://www.oxfordbiblicalcstudies.com/article/opr/t94/e1268 (accessed May 19, 2013).