Agate is the name given to a group of aggregates of various forms of silica, chiefly chalcedony. According to legend, the agate (achates) was named for the river Achates, now the Drillo, in Sicily, where the stone was first found. They form in nodules or holes in ancient lava flows. Water slowly percolates through the nodule depositing silica from the cooled lava in regular layers. Sometimes the silica contains minerals, when this happens a banding effect is visible. In different specimens the banding stops meaning the deposition has not proceeded far enough to fill the cavity resulting in a hollow space. The final deposit usually consists of quartz, often amethyst-having apices of the crystals directed toward the free space, this is called a geode. Agate can form in various patterns depending on the minerals and the time they were deposited.

Stabilizing, Strengthening.