Zech. 7:12 - "They made their hearts diamond-hard lest they should hear the law and the words that the Lord of hosts had sent by his Spirit through the former prophets. Therefore great anger came from the Lord of hosts."

There are several places in the Biblical text where the value of precious stones is used to communicate something of the value of heavenly things. Think of all the precious stones John saw in Revelation to describe the dazzling glory of being in God's presence (Rev. 4:1-3, 21:9-27). Jesus compared the kingdom of heaven to a pearl of great price worth more than anything else you may have (Matt. 13:45-46). Paul even compared some of the Christians he taught with precious material's ability to withstand the refining fire of judgment (1 Cor. 3:12-15). Even though the Bible mostly uses gemstones and other valuables to hint at the worth of heavenly things, in Zechariah 7, the comparison is negative.