Short answer: agate is a banded variety of chalcedony, the fine-grained, fibrous form of quartz. Its bands form inside cavities in rock, usually volcanic rock, and no one has ever made agate banding in a lab. It comes in countless varieties, from pale blue lace agate to crazy lace, Botswana and Lake Superior agate. It rates 6.5 to 7 on the Mohs scale and has been carved and worn for more than 4,000 years. Many brightly colored agates are dyed. In crystal traditions it is a balancing stone and the traditional zodiac stone for Gemini.
What is agate?
Agate is chalcedony, silicon dioxide in tiny fibers, laid down in alternating bands of different colors, often with some visible quartz crystals. Most agates are banded in concentric rings that follow the shape of the cavity, called fortification or wall banding. Some have flat, parallel bands, called level banding.
Not every stone called agate is a true agate. Moss agate and fire agate are chalcedony without bands, and so are not strictly agates. And not every banded chalcedony is an agate: banded chert, for example, is not.
How does agate form?
Geologists understand the early stages, but how the bands develop is still debated. Agate forms hidden inside rock, so it cannot be watched forming, and its banding has never been reproduced in a laboratory.
Most agates form as nodules in volcanic rock. In basalt and andesite, they fill gas bubbles called vesicles; the walls are often coated with a thin green mineral layer, which gives agate "husks" their pitted surface. Silica-rich water seeps in and forms a silica gel, which crystallizes into fibrous chalcedony, layer by layer. Agates also form in veins in many kinds of rock, including the lace agates, and in fossils where silica has replaced the original material.
Agates are much harder than the rock around them, so they survive after the host rock weathers away. That is why some, such as Lake Superior agates, are often found loose on beaches and in gravel.
Why are agates different colors?
Iron causes the color of most agate bands: microscopic spheres of hematite give red bands and of goethite yellow ones. Manganese, chromium and nickel color some bands. Traces of uranium make a few agates glow green under short-wave ultraviolet light.
What are the main types of agate?
- Blue lace agate: a pale blue and white lace agate formed in veins, mainly from Namibia, with smaller sources in Malawi, Kenya, Turkey, Georgia and Zambia.
- Crazy lace agate: a tightly swirling, multicolored lace agate from Mexico.
- Botswana agate: nodules from Botswana with contrasting bands of purple, pink, black, gray and white.
- Lake Superior agate: iron-colored agate in red, orange and white from the Lake Superior region of the United States and Canada. Formed in lava flows about 1.1 billion years ago, it is believed to be the oldest agate in the world, and it has been Minnesota's state gemstone since 1969.
- Brazilian agate: large nodules from Rio Grande do Sul and nearby states. Naturally they are often pale yellow, gray or colorless, so most are sold dyed.
- Condor agate: bright red and yellow fortification agate from Mendoza, Argentina.
- Dendritic agate: fern-like inclusions of manganese or iron oxides on or between the bands.
- Moss agate: green or brown moss-like inclusions. It was once thought to be petrified moss; the "moss" is actually minerals such as celadonite, hornblende or chlorite.
- Onyx and sardonyx: onyx is agate with straight bands, typically black and white; sardonyx has red to brown bands with white or black. The name onyx comes from the Greek for fingernail.
Agate geodes
Many geodes have a shell of agate lining a hollow center of quartz crystals. Sliced and polished, they show both: agate bands around the outside and sparkling crystals within.
A stone with a very long history
Agate, especially moss agate, was used in the Stone Age for tools such as arrow and spear points, needles and hide scrapers. The Natufian people of the Levant made knives and arrowheads from moss agate as early as 10,000 BCE. Agate jewelry from Sumer dates to about 2500 BCE, and the ancient Egyptians, Mycenaeans and Romans all wore it. In ancient Greece it was used in jewelry and in the seal stones of warriors.
The name goes back to the Greek writer Theophrastus. He and the Roman naturalist Pliny the Elder said it was named after the River Achates in Sicily, though etymologists think the river was more likely named after the stone.
From about 1375 CE, the town of Idar-Oberstein in Germany worked agate on an industrial scale. When its local supply ran short, it imported Brazilian agate, arriving as ships' ballast, and developed chemical processes to color it, selling dyed agate beads around the world.
Is agate dyed?
Very often. Many duller agates sold commercially are treated to enhance their color. Brightly colored agate geodes in pink, teal or electric blue are almost always dyed, and the same goes for many vivid slices and beads. Dyed agate is still real agate; it should simply be sold as dyed.
Signs of dye: colors that are too vivid, and color concentrated sharply in some bands more than others, with a striped or "candy" look.
What is agate traditionally associated with?
In modern crystal traditions, agate is associated with balance, stability and harmony, a meaning many link with its even, layered bands. Blue lace agate is associated with calm and gentle communication and is commonly linked with the throat chakra. Agate is the traditional zodiac stone for Gemini in the list published by George F. Kunz in 1913.
These are traditions and beliefs. They are part of why people choose agate, not properties you can measure in the stone.
Raw, tumbled and polished agate
Agate takes a bright polish and is one of the most common tumbled stones, often tumbled together with jasper. It is sold as rough nodules, polished halves and slices, spheres, towers, palm stones and carvings.
Agate jewelry
At 6.5 to 7 on the Mohs scale, agate is durable for beads, cabochons, rings and pendants. Its use in jewelry goes back thousands of years, and in ancient Greece agate was carved into the seal stones of warriors.
How do you care for agate?
- Wash it with warm water and mild soap, then dry it.
- Store it apart from harder gems such as sapphire and topaz.
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Frequently asked questions
What is blue lace agate?
A pale blue and white banded agate that forms in rock veins, mainly from Namibia. In crystal traditions it is associated with calm communication and the throat chakra.
Is agate a type of quartz?
Yes. Agate is banded chalcedony, a fine-grained, fibrous form of quartz.
Is brightly colored agate natural?
Usually not. Vivid pink, teal and electric blue agate is almost always dyed. Natural agate colors come mostly from iron, giving reds, oranges and yellows, along with white, gray, brown and pale blue.
What is moss agate?
Chalcedony with green or brown moss-like mineral inclusions. Despite its name, it is not strictly an agate, since it lacks bands.
What is agate traditionally associated with?
In crystal traditions it is associated with balance and harmony, and it is the traditional zodiac stone for Gemini. These are traditional meanings rather than measurable properties.
Where is agate found?
Worldwide. Well-known sources include Brazil, Mexico, Botswana, Namibia, Argentina and the Lake Superior region of North America.
Sources
- Wikipedia, Agate: https://en.wikipedia.org/wiki/Agate
- Wikipedia, Lake Superior agate: https://en.wikipedia.org/wiki/Lake_Superior_agate
- Wikipedia, Birthstone (zodiac stones after Kunz, 1913): https://en.wikipedia.org/wiki/Birthstone