Agate (/ˈæɡɪt/ AG-it) is a variously translucent, banded variety of chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and may also include visible quartz crystals. They are common in nature and can be found globally in a large number of different varieties. There are some varieties of chalcedony without bands that are commonly called agate (moss agate, fire agate, etc.); however, these are not true agates. Moreover, not every banded chalcedony is an agate; for example, banded chert forms via different processes and is opaque. Agates primarily form as nodules within volcanic rock, but they can also form in veins or silicified fossils. Agate has been popular as a gemstone in jewelry for thousands of years, and today it is also popular as a collector's stone. Some duller agates sold commercially are artificially treated to enhance their color.

Agate was given its name by Theophrastus, a Greek philosopher and naturalist. He discovered the stone c. 350 BCE along the shoreline of the River Achates (Ancient Greek: Ἀχάτης), now the Dirillo River, on the Italian island of Sicily, which at the time was a Greek territory.

Agate is composed principally of chalcedony, a microscopic (microcrystalline) and submicroscopic (cryptocrystalline) form of quartz that grows in fibers. The chemical composition of quartz is SiO₂, also known as silica. Normally, between 1% and 20% of the "quartz" in chalcedony is actually moganite, a quartz polymorph. Unlike macroscopic (macrocrystalline) quartz, which is anhydrous, chalcedony normally contains very small amounts of water bound to its crystal structure.

Agate contains multiple layers, or bands, of chalcedony fibers. The fibers are twisted, forming a helical shape. There are two different types of chalcedony fibers: length-fast and length-slow, also known as quartzine. Agate primarily contains length-fast chalcedony fibers, consisting of crystals stacked perpendicular to the c-axis (side to side). Some intergrown quartzine may also be present, consisting of quartz crystals stacked parallel to the c-axis (tip to tip).

Agate can sometimes contain small amounts of opal, an amorphous, hydrated form of silica. Agates also frequently contain macrocrystalline quartz, particularly in the center.

Geologists generally understand the early stages of agate formation, but the specific processes that result in band development are widely debated. Since they form in cavities within host rock, agate formation cannot be directly observed, and agate banding has never been successfully replicated in the lab.

Agates are most commonly found as nodules within the cavities of volcanic rocks such as basalt, andesite, and rhyolite. These cavities, called vesicles (amygdaloids when filled), are gas bubbles that were trapped inside the lava when it cooled. The vesicles are later filled with hot, silica-rich water from the surrounding environment, forming a silica gel. This gel crystallizes through a complex process to form agates. Since agates usually form in lavas poor in free silica, there are multiple theories of where the silica originates from, including micro-shards of silica glass from volcanic ash or tuff deposits and decomposing plant or animal matter. Agates are much harder than the rocks they form in; some varieties (e.g. Lake Superior agates) are frequently found detached from their host rock.

In wall-banded agates, chalcedony fibers grow radially from the vesicle walls inward, perpendicular to the direction of the bands. The vesicle walls are often coated with thin layers of celadonite or chlorite, soft, green phyllosilicate minerals that form from the reaction of hot, silica-rich water with the rock. This coating provides a rough surface for the chalcedony fibers to form on, initially as radial spherulites. The rough surface also causes agate husks to have a pitted appearance once the coating has been weathered away or removed. Sometimes, the spherulites grow around mineral inclusions, resulting in eyes, tubes, and sagenitic agates.