Trona, TROH nuh, is a gray, white, or yellowish-white mineral that contains sodium carbonate, sodium bicarbonate, and water. It forms from ground water that dissolves sodium in rocks. Some of the sodium-rich water pools underground or on Earth's surface. As the water evaporates, trona crystals form. The crystals appear as fibrous (threadlike) or columnar (columnlike) structures in rock layers or in thick beds deposited by modern or ancient saltwater lakes. Deposits near the surface occur only in dry areas because rainwater rapidly dissolves trona. .