Agate is a type of chalcedony, a milky form of quartz, that appears in a striking banded formation which people have found aesthetically pleasing for centuries. Chemically, it is identical to quartz, amethyst, and carnelians, but because it forms in a different way, agate looks radically different. Like all forms of chalcedony, the chemical formula for an agate is SiO2, and the colors come from impurities in the silicon dioxide. In addition to coming in a range of stunning colors in nature, the highly porous stone is also sometimes dyed to be more vivid in color.

Brazil, Uruguay, and the Western United States are all sources for raw agate. Traditionally, Germany has been the capital of agate processing, and agate products are shipped from Germany to jewelers and stores all over the world. The distinctive stone is used in jewelry and to make decorative household ornaments such as bookends and coasters. Some religious ornaments are made from agate, such as worry beads and meditation stones carried in many cultures.

Deposits of volcanic rock frequently harbor agate, which is formed when silicates make deposits in cracks and holes in rock. The outer layer of an agate tends to turn dark and crusty, but the layers inside form rich bands of color which can be cut in cross section and polished for use in jewelry. Agate is very similar to onyx, another form of chalcedony which forms in successive layers, but agates tend to have more irregular bands and a wider range of colors.

Some agates are so distinctive that they carry their own names, such as moss agate, a unique form which has feathers of rich mossy green riddling a white agate. Iris agate, another special occurrence of the rock, has a remarkable iridescent sheen. Both forms, along with other varieties, are used in rings, bracelets, and necklaces, and are usually cut large to show off the rich bands of color in the stone. Often, a piece of agate jewelry will have an unusual shape, to accommodate a unique and striking pattern in the stone.

If kept in heat and sunlight, agate can crack and fade, so make sure to store it in a cool, dark place. If agate becomes dirty, it can be washed in soap and a mild detergent solution, but do not use harsh or abrasive chemicals on it. Agates can scratch or crack if they are handled roughly, so be gentle with agate jewelry, and try to avoid slamming it into things or dragging it across hard surfaces.