8 Now, what are the stones of fire, but the children of Zion and the children of Jerusalem? For in the ancient time, in the days of David and of Solomon his son, Hiram was a friend to those of the house of Israel. But when they were carried away captive from their place, he rejoiced over them and spurned them with his feet, and did not remember the friendship of the house of David. And as to that which I said that the children of Judah were called the stones of fire, it was not of my own thought that I said it, but Jeremiah the Prophet spoke concerning them; for when he was calling forth tears for them in the Lamentations, he said:— The children of Zion were more excellent than precious stones. And again he said:— How are the stones of the Sanctuary cast down at the head of all the streets? And again He said by the Prophet:— The stones were holy that were cast down in his land. And as to these very stones, the fire was burning in them, as Jeremiah said:— The word of the Lord became in my heart like burning fire and it was hot in my bones. And again He said to Jeremiah:— Lo! I give My word in your mouth as fire, and this people shall be as wood. And again He said words shall go forth as fire, and as iron that cuts the stone. On this account the Prophets, among whom Hiram the Prince of Tyre was walking, were called stones of fire.
Source: Demonstrations (New Advent)