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.

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