On heating, borax first loses its water of crystallization and swells up. Upon further strong heating, it melts to form a transparent liquid which solidifies into a glassy bead. This bead consists of sodium metaborate ($NaBO_2$) and boric anhydride ($B_2O_3$). The balanced chemical equation for the decomposition of anhydrous borax is: $Na_2B_4O_7 \xrightarrow{\Delta} 2NaBO_2 + B_2O_3$ Therefore, the product 'X' is $B_2O_3$.