The compound for “corn” also contains 玉:
玉蜀黍 (tōmorokoshi: corn) jewel + green caterpillar + millet
As you may have noticed, the breakdown is a little strange! Moreover, 玉 has the reading TŌ here, which doesn’t match the official yomi of tama and GYOKU. That’s because the above compound appears to be ateji.