Steel
Srimad-Bhagavatam
SB Canto 1
Before the Battle of Kurukṣetra, Dhṛtarāṣṭra's policy was peaceful annihilation of his nephews, and therefore he ordered Purocana to build a house at Vāraṇāvata, and when the building was finished Dhṛtarāṣṭra desired that his brother's family live there for some time. When the Pāṇḍavas were going there in the presence of all the members of the royal family, Vidura tactfully gave instructions to the Pāṇḍavas about the future plan of Dhṛtarāṣṭra. This is specifically described in the Mahābhārata (Ādi-parva 114). He indirectly hinted, "A weapon not made of steel or any other material element can be more than sharp to kill an enemy, and he who knows this is never killed." That is to say, he hinted that the party of the Pāṇḍavas was being sent to Vāraṇāvata to be killed, and thus he warned Yudhiṣṭhira to be very careful in their new residential palace.