Agate is a banded form of chalcedony, the microcrystalline variety of quartz. It forms when silica-rich water fills a cavity in rock and deposits layer after layer of tiny quartz fibers. The result is a tough, easily polished stone with curved or straight bands in almost any color. Agate has been carved into seals, beads and bowls for thousands of years, and it remains one of the most widely used ornamental stones in the world.
What agate is made of
Agate belongs to the chalcedony family. Under a microscope, chalcedony is a mass of quartz fibers far too small to see with the eye, packed together with a little of a related silica mineral called moganite and some trapped water. Because the crystals are so fine, agate has no visible crystal faces and breaks with a smooth, shell-like fracture.
What separates agate from plain chalcedony is the banding. Each band records a change in the silica supply, the impurities in the water, or the size of the fibers. Iron oxides give red, orange and brown bands. Pure layers look white, gray or bluish. Some bands are milky and others nearly transparent, which gives good agate its depth when light passes through a thin slice.
Agate is closely related to carnelian, which is orange-red chalcedony, and to onyx, which has straight, parallel bands. Opaque, impurity-rich relatives are sold as jasper.
How agate bands form
Most agate grows inside gas bubbles (vesicles) left behind in cooling lava, especially basalt and rhyolite. Groundwater carrying dissolved silica seeps into these empty spaces over long periods. Silica coats the walls first, and the deposit grows inward toward the center. The igneous rocks of southern Brazil and Uruguay are full of these filled cavities, and they supply much of the agate on the market.
Geologists still debate the exact process that makes such regular bands. Leading explanations involve silica gel that organizes itself into layers as it hardens, or repeated pulses of silica-rich fluid. Either way, the result falls into two common patterns:
- Fortification agate has concentric bands that follow the shape of the cavity, like the walls of a fort seen from above.
- Water-level or onyx-banded agate has flat, parallel layers that settled under gravity. These stones show which way was "up" when they formed.
Many agates have a hollow center lined with quartz crystals. When the crust of crystals is fine and sparkly, the trade calls it druzy. Thunder eggs from Oregon are rhyolite nodules with an agate or chalcedony filling, and geodes are similar cavities with a larger open space.
Types of agate
Hundreds of local and trade names exist. These are the types you will meet most often.
Fire agate
Fire agate is brown chalcedony with thin layers of iron oxide (limonite or goethite) grown into it. Light reflecting from these layers produces flashes of orange, green, gold and sometimes violet, an effect described on the page about gemstone optics. It comes from Mexico and the southwestern United States, especially Arizona. Cutters grind away the brown chalcedony by hand, following the iridescent layers, so most fire agate ends up as free-form cabochons.
Moss agate
Moss agate usually has no banding at all. It is translucent chalcedony with green, brown or black inclusions that branch like moss or underwater plants. The inclusions are minerals such as chlorite or iron and manganese oxides. India has long been a major source.
Dendritic agate
Dendritic agate has fern-like or tree-like black markings, usually manganese oxide, that grew along tiny cracks. The patterns look organic, but they are mineral growths. Thin slices with a single clear "tree" are popular for pendants.
Lace agate
Lace agate has tight, looping bands that look like lace. Blue lace agate, a pale blue and white stone, is best known from Namibia. Crazy lace agate, with swirling cream, red and brown bands, comes from Chihuahua, Mexico.
Other well-known names include Botswana agate (fine gray, pink and white bands), Lake Superior agate (red and orange, found in glacial gravels of the upper Midwest) and Mexican Laguna agate, which collectors prize for very tight, colorful banding.
Dyeing agate
Natural agate tends toward gray, white, tan, red and brown. The bright blues, purples, pinks and greens sold in shops are almost always dyed. Agate takes dye well because the spaces between its quartz fibers absorb liquids, and some bands absorb more than others, which makes the banding stand out.
The practice is old. Roman writers described soaking agate in honey and heating it to darken it. The method later known as sugar and acid treatment works the same way: stones soak in a sugar solution and are then treated with sulfuric acid, which carbonizes the sugar inside the stone and turns it black. Much of the black "onyx" in jewelry is agate treated this way. Iron salts and heat give reds, while other metal salts and modern organic dyes give blues and greens.
Dyeing is accepted in the trade and is stable for most uses, but it should be disclosed. Our overview of gemstone treatments explains disclosure norms. Signs of dye include colors that do not occur in nature, color concentrated along cracks, and bands that look unusually even and saturated.
Agate and Idar-Oberstein
The towns of Idar and Oberstein in western Germany grew into a major stone-cutting center because of local agate. Records show agate working in the area by the 1500s. Water from the Nahe River and its streams powered large sandstone grinding wheels, and workers lay on their stomachs on wooden benches to press stones against the turning wheels.
By the early 1800s local deposits were running out. Emigrants from the region found rich agate in Rio Grande do Sul in southern Brazil, and shipments of Brazilian rough began reaching Germany. Idar-Oberstein cutters also refined the dyeing methods described above. The history of the town is covered on the Idar-Oberstein gem cutting page.
Uses of agate
Agate's toughness and fine grain make it good for carving. Ancient cultures cut it into beads, seals and amulets, and Renaissance workshops turned it into cups and bowls. Layered agate is the classic material for cameos, where the carver cuts a light top layer into a figure against a dark background.
Its hardness also gives it practical uses. Agate mortars and pestles are still used in laboratories, and agate bearings and burnishing tools have a long history. Today you will see agate as cabochons, beads, slices, bookends, coasters and carvings.
Buying and caring for agate
Judge agate by its pattern, color and translucency. Sharp, fine bands, clean contrast, an interesting picture (in moss or dendritic agate) and a good polish all add appeal. Natural colors and unusual locality material sell for more among collectors than dyed stones.
Agate is about 7 on the Mohs hardness scale, has no cleavage and resists chipping well, so it suits rings and bracelets. Clean it with warm soapy water and a soft brush. Dyed agate can fade if left in strong sunlight for long periods, and harsh chemicals may lift surface dye.
Frequently asked questions
Is agate a type of quartz?
Yes. Agate is banded chalcedony, and chalcedony is microcrystalline quartz. It has the same chemistry as rock crystal (silicon dioxide) but is made of tiny fibers instead of one large crystal.
Is blue agate natural?
Pale blue lace agate is natural. Bright, saturated blue, purple or pink agate is almost always dyed.
What is the difference between agate and onyx?
Agate usually has curved bands, while onyx has straight, parallel bands. Most black onyx sold in jewelry is agate or chalcedony that has been dyed black.
Where is agate found?
Agate occurs worldwide, mostly in volcanic rocks. Brazil, Uruguay, Mexico, India, Botswana and the United States are long-standing sources.