Agate: Properties, Facts and Photos
Agate is a banded variety of chalcedony known for its wide range of colours and patterns. This article explains how it forms, the different types and where their names come from, what ancient writers said about it, and why so much agate is treated or dyed.
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Contents 1. What is Agate? |
What is Agate?
Agate is a translucent variety of chalcedony made up almost entirely of silicon dioxide.
Although it often occurs in white, grey, brown, black and other muted tones, it can also be exceptionally colourful. Its colour comes from trace minerals and microscopic inclusions, while its distinctive banding is formed by successive layers of chalcedony that differ in colour, translucency or texture.
Quartz crystals in agate are colourless in their purest form, but turn red or brown with the presence of iron. With manganese, the crystals turn pink, and with chromium, a shade of green.
The easiest type of agate to identify often has distinctive bands. Some argue that a stone must exhibit banding to be classified as a 'true agate.' However, several types of agate are not banded.
Over the years, many discussions have taken place regarding the use of trade names. From a geological perspective, they're often inaccurate and misleading.
Moss and dendritic agate do not exhibit visible banding but feature inclusions known as dendrites. Often mistaken for organic matter, dendrites are crystal formations similar to those on glass during winter.
The word dendrite comes from the Greek word for 'tree' and refers to a 'branching form.'
How Agate Forms
Although commonly found in igneous rocks, agate can also occur in other rock types. Its formation is highly complex, starting when groundwater containing silica (such as sand or quartz) enters the rock through cavities and the silica gradually precipitates from the solution, building up on the cavity walls.
As the process repeats over time, layers of minerals build up within the rock, following the outline of the cavity.
Once full, the mass begins to crystallise. Quartz crystals often grow in space that has not been filled.
The cavity containing the crystallised minerals is known as a nodule. Depending on the type of rock, it can be incredibly resilient to weathering. It's not unusual for the nodule to survive long after the host rock has eroded.
The agate formation remains locked away until the nodule is found and broken open.
On the Mohs scale of mineral hardness, agate grades 6.5 - 7. Being relatively hard makes it quite easy to work with.
The History of Agate
The history of agate can be traced back more than 2000 years to the writings of ancient Greek philosopher Theophrastus [c.370-285 BC].
The process of describing different types of agate and their properties and meanings can be found in his works, Theophrastus On Stones.
In this reference, he compares the hardness of agate to onyx and discusses how one type of agate differs from the other because of "its irregular and uncertain manner of spots, clouds and variegations".
He says although agate stones are predominantly grey, different colours can be present and are often beautifully spread out. Patterns may resemble trees, shrubs, plants, clouds, rivers and even animals.
He goes on to say, "the ancients" organised agates into different types. Each one was given a name that highlighted its difference from the "common agate". That difference could be colour, markings or texture.
Red agate was known as haemachates. The prefix 'haema' comes from the Greek for 'blood', which refers to the stone's colour. The name 'achates' came from the river in Sicily where agate was discovered.
Regarding its discovery, Theophrastus wrote that these stones have been "found to exist in almost every nation on Earth."
Those with patterns resembling trees and shrubs were called dendrachates (from dendrite), whilst other types were "idly named" according to the powers they held or because of their similarity to animal skin.
The ancient Roman author Pliny the Elder also wrote about the properties and meaning of agate in his works 'Naturalis Historia'.
Pliny wrote;
"Achates was a stone formerly in high esteem, but now held in none. It was first found in Sicily, near a river of that name; but has since been discovered in numerous other localities. In size it exceeds any other stones of this class, and the varieties of it are numerous, the name varying accordingly."
He goes on to name several types of agate and explains where they can be found. Like Theophrastus, Pliny also makes reference to dendrachates (dendritic agate) which he says is "marked with small shrubs."
He discusses the meaning of different types of agate from India and their distinctive markings and says, "The sight of some is beneficial for the eyes whilst others when held in the mouth allay thirst. Then there are those which are good for counteracting the effects of poison inflicted by the scorpion. Agate stone can also be found in Cyprus."
Pliny believed those with uniform colour would render athletes invincible and said agate with the highest value has the transparency of glass.
Agate, one of the oldest known and worked stones, was highly valued by many ancient cultures, including the Sumerians, Egyptians, Greeks, and Romans. It was widely used for talismans, seals, vessels (containers for liquids), beads and gemstones.
Different Types of Agate
Agate stones are typically characterised by concentric, curved or angular banding. They can also exhibit other curious and intricate patterns.
Hundreds of different types of agate can be found around the world, and many have been given trade names. These often indicate the locality where a stone is found or a particular colour or characteristic.
Banded Agate
Banding is the feature most people associate with agate, and it forms as successive layers of chalcedony build up inside a cavity. Each layer differs slightly in colour, translucency or texture, which is what makes the bands visible.
Two arrangements are common. In most agates, the bands follow the shape of the cavity wall, curving around it in concentric layers. In others, the bands are level across the stone, and formed as material settled rather than lining the cavity.
Not every agate is banded. Moss and dendritic agate are two well-known exceptions, which is why some people argue they shouldn't be classed as agate at all.
Fortification Agate
Fortification agate is a generic name for stones whose bands are arranged at sharp angles. The name stems from the shape, which was once believed to resemble the lines of a fortress. A fortification is a defensive wall or reinforcement built to strengthen a building against attack.
It's one of the most common forms, and several better-known agates are types of fortification agate, including blue lace and crazy lace.
Blue Lace Agate
A pale blue banded agate with delicate, lacy patterning. The finest material comes from Namibia.
Crazy Lace Agate
Crazy lace agate is known for its tightly folded, swirling bands, often in reds, oranges, creams and browns. The pattern is far more contorted than in most fortification agates, which is where the name comes from. Most material on the market comes from northern Mexico.
Botswana Agate
Botswana agate comes from Botswana and is known for its very fine, closely spaced banding, usually in greys, pinks and soft browns. The bands are often so narrow and regular that the stone looks almost striped.
Brazilian Agate
Brazil is one of the world's largest sources of agate, and its nodules can be very large. The natural material is typically grey, white or pale brown with broad banding.
It's also the material most often used for the brightly coloured agate slices sold as bookends, coasters, and other decorative items. These stones are often dyed.
Fire Agate
Fire agate exhibits a distinctive flash of colour caused by the interference and diffraction of light in thin layers of silica and iron oxide. It's found mainly in the southwestern United States and northern Mexico.
Moss and Dendritic Agate
Neither shows visible banding. Instead, they contain branching inclusions that resemble organic matter trapped within the stone. Despite the appearance, these patterns are mineral formations known as dendrites.
Polka Dot Agate
Polka Dot Agate is often, but not always, spotted. It comes from central Oregon in the United States.
Is Agate Dyed?
Agate is one of the most frequently dyed stones on the market. Its microscopic porosity allows it to absorb dye well, while its banding creates attractive coloured patterns. Polished slices, bookends and ornamental stones with distinctive blue, green, pink or purple colouring will have been dyed.
Agate's natural colour depends on the presence of trace minerals and other impurities. Pale white and grey agate is frequently used for dyeing. Nearly all commercial black onyx is agate that's been sugar-treated and heated in acid.
The practice of dyeing rocks and minerals is not new. It can be traced back thousands of years. The Stockholm Papyrus, a Greek manuscript compiled in Egypt around the third or fourth century AD, contains practical recipes for making cheap stones look like precious gems. It was published as 'Papyrus Graecus Holmiensis' in 1913.
Several of these recipes explain how rock crystal could first be 'opened' or made more porous, then coloured using substances including copper compounds and organic colourants. The treatments were intended to produce imitations of emerald, ruby and other valuable stones.
Writing in the first century AD, Pliny the Elder complained that books were circulating with instructions for staining quartz to imitate emerald, and remarked that no other fraud yielded larger profits.
Agate is often treated to mimic the reddish-orange colour of carnelian, which can be more expensive. Rather than being dyed, the stone is heated to oxidise its iron content. It may first be impregnated with an iron compound to intensify the colour.
More Agate Facts
An agate was believed to have been the second stone in the third row of the high priest breastplate. This religious garment from biblical times was adorned with twelve gemstones. It was worn by the Jewish high priest whilst presenting himself to God.
Despite everything that has been learned about the formation of agate, so much is still not fully understood. Its formation has never been studied from start to finish in real time. Nor has it been possible to recreate agate in a laboratory setting. Therefore, much of what is known about its formation is supposition.
Agate is associated with the star sign Gemini. On the Traditional and Ayurvedic Birthstone Chart, it's the birthstone for May. On the Mystical Chart, agate is the birthstone for September.
Article Pictures
The agate at the top of our article is courtesy of Captain Tenneal. The second (dendritic inclusions in agate), fifth (crazy lace agate) and last picture are courtesy of Stan Celestian.
The agates in the museum display are in the Smithsonian Museum of Natural History, Washington, D.C. Photo: Stone Mania.
The fourth picture (dendritic agate): Courtesy of Steve Blyskal.
Pop-up photos
Blue lace agate: Stone Mania. Fortification agate: Courtesy of Steve Blyskal. Fairburn agate, fire agate: Courtesy of Captain Tenneal. Botswana agate: Courtesy of Stan Celestian.
