Agate

Agate is a banded variety of chalcedony, the microcrystalline (cryptocrystalline) form of quartz. It is defined by its translucent layers and bands, which can form concentric rings, fortification patterns, or wavy lace-like designs in an enormous range of colors. Many agates are naturally muted and are commonly dyed to enhance color.

Agate fills cavities in volcanic and sedimentary rocks, often as the lining of nodules and geodes. There are countless named varieties based on color, pattern or locality (such as fortification, moss, lace and fire agate).

Valued since antiquity, agate is tough, takes a brilliant polish, and is one of the most popular lapidary materials in the world.

Agate forms when silica-rich solutions deposit successive layers of chalcedony inside cavities, gas bubbles (vesicles) and cracks, typically in volcanic rocks such as basalt and rhyolite, and also in some sedimentary rocks. Each band records a stage of deposition, with varying trace impurities producing different colors.

The layers build inward from the cavity walls, sometimes leaving a hollow center that may form a crystal-lined geode. Agates are found worldwide; major sources include Brazil, Uruguay, India, Mexico, Botswana, Germany (Idar-Oberstein) and the Lake Superior region of the USA.

Agate is translucent banded chalcedony with a waxy to vitreous luster, hardness 6.5-7 (scratches glass and steel), no cleavage, conchoidal fracture and a white streak. The defining feature is its layering, especially concentric or fortification banding visible when cut.

Look-alikes: Jasper is opaque (agate is at least partly translucent) and usually unbanded. Chalcedony without bands is simply called chalcedony. Carnelian and onyx are agate-family chalcedonies distinguished by color and banding style. Dyed agates show unnaturally vivid, even colors. Translucency plus banding distinguishes agate from opaque jasper.

Agate is one of the most widely used lapidary stones: cabochons, beads, slices, bookends, coasters, knife handles, mortars and pestles, and carvings. Polished agate slices and geodes are popular decor. Historically it was used for seals, cameos, beads and burnishing tools.

It is generally inexpensive, though fine patterned or rare locality agates (like Lake Superior or condor agates) can be collectible and valuable. Much commercial agate is dyed to enhance its color.

Metaphysically agate is associated with stability, grounding and protection, with properties varying by color and type, though such claims are not scientifically supported.