The rare Hebrew word eqdach refers to a sparkling, burning gem or fire-coal — often translated 'carbuncle,' 'sparkling jewel,' or 'crystal.' It appears in Isaiah's vision of the New Jerusalem, describing the radiant building materials God will use to restore His people.
Isaiah's use of eqdach in the promise of New Jerusalem's restoration speaks to the blazing beauty of God's future dwelling among His people. Where the ancient city had been destroyed and its people humiliated, God promises a city whose very foundations burn with divine glory. This anticipates the New Jerusalem of Revelation 21, where the city's foundations are adorned with every kind of precious stone.