וְשַׂמְתִּ֤י כַּֽדְכֹד֙ שִׁמְשֹׁתַ֔יִךְ וּשְׁעָרַ֖יִךְ לְאַבְנֵ֣י אֶקְדָּ֑ח וְכָל־גְּבוּלֵ֖ךְ לְאַבְנֵי־חֵֽפֶץ׃
Malbim on Isaiah
Kadkhod is the gemstone called Shoham, according to the Sages.
Ibn Ezra on Isaiah
כדכד Agates. It is hap. leg.; it is the name of a precious stone.