Agates are a type of chalcedony, which is a cryptocrystalline quartz. Agates grow in the voids of rock masses such as lavas, building themselves in layers from the outside-in over millions of years. As the softer, surrounding rock erodes away, the agates remain. Sometimes these agates are themselves eroded and smoothed into pebbles in riverbeds. Other times they can be found as solid nodules that can be sliced and policed, or hollow geodes whose interiors are studded with jagged crystals.
The most common types of nodular agates are marked by distinct, concentric bands. The differences in the colors of the bands in a given agate are due to the impurities in the silica, the basic mineral of the forming rock. Dendritic or landscape agates refer to white gemstones whose inclusions, usually iron-based, resemble a tree or bush growing throughout the stone. Landscape agates are usually cut and shaped so that one of these trees is centered within the finished gem.
The inclusions in moss agates are finer still, caused by manganese (to produce blacks and browns) and chloride (shades of mossy green). And when polished, fire agates reveal clusters of mineral bubbles that appear suspended within the iridescent jewel.
Two other types of agates are created when silica replaces organic materials rather than filling rocky voids. Petrified or agatized wood is formed when wood that has been surrounded by sediments and deprived of oxygen, thus preventing its outright decomposition, is replaced by silica that slowly takes its shape. Agatized coral is formed in much the same way, although these stones are far more rare.
The use of agates in jewelry dates to the end of the stone age; neolithic agate beads believed to be 9,000 years old have been found in Anatolia, present-day Turkey. Thought to provide protection from the evil eye, agates are frequently sliced, cut, and polished into cabochons for rings, bracelets, pins, and pendants. The stone is also carved into cameos.
Art Nouveau and Arts and Crafts jewelers favored agates over traditional gemstones like diamonds, rubies, and sapphires because they appeared more naturalistic. That said, because agates are relatively porous, they are routinely dyed to accentuate their natural colors, thus adding even more differentiation and drama to their naturally occurring bands. In principle, jewelers should disclose whether or not an agate has been dyed, but one way to tell is to put a stone in a sealed plastic bag—when it’s hot out, the agate will sweat out some of its dye.