Francis Brown et al. interprets גִּלָּיוֹן as a reference to "tablets of polished metal."
Francis Brown, Samuel Rolles Driver, and Charles Augustus Briggs, Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (Oxford: Clarendon Press, 1977), 163 (Logos ed.)
†גִּלָּיוֹן S1549 TWOT350c GK1663 n.m. table, tablet (Talm גִּלְיוֹן, the empty margin of page or roll, vid. LagGN 1881, 403, 199)—קַח־לְךָ גִּלָּיוֹן גָּדוֹל וּכְתֹב עָלָיו take thee a great tablet and write upon it Is 8:1; pl. הַגִּלְיֹנִים tablets of polished metal, mirrors Is 3:23 𝔗 𝔙 Ges Che Di De; but transparent garments, gauzes, 𝔊 Ew (cf. Arabic جَلْوَةٌ (jalwatun) fine garment).