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Agate Stone | Meaning and Properties

 

 

Agate stone with colourful banding. A museum exhibit on a blackened background

 

 

Contents

1. History of the Mineral Agate
2. What Exactly is Agate?
3. Different Agate Stones
4. A Few More Facts
5. Article Photos
6. Our Collection of Agate
7. Read More

 

 

 

History of the Mineral Agate


Agate is one of the oldest of all minerals and references to this popular stone can be traced back more than two thousand years to the writings of Theophrastus the Ancient Greek philosopher (c.370-285 BC).  The process of describing different varieties of agate can be found in his works Theophrastus On Stones in which he compares the hardness of agate to onyx and talks about how one stone differs from the other because of "its irregular and uncertain manner of spots, clouds and variegations".  He says that although 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 separated agate stones into different varieties giving each one a name that highlighted its difference from the "common agate".  That difference could be colour, markings or texture.

Red agate was called haemachates the prefix "haema" coming from the Greek word for blood with reference to the stone's colour.   The name achates as agate was known at that time came from the name of a river in Sicily where agate was first discovered.  In reference to the discovery Theophrastus wrote; these stones have been found to exist in almost every nation on Earth, those which featured patterns resembling trees and shrubs were called dendrachates (from the word dendrite).  Other varieties were "idly named" according to the powers they held or even because of their similarity in colour to animal skin. 

Agate stones were highly valued by many ancient cultures including the Sumerians, Egyptians, Greeks and Romans.  They were used extensively for talismans, seals, vessels (container used mainly for liquids), beads and gemstones.  The Ancient Roman author, naturalist and natural philosopher Pliny the Elder (23 AD - 79 AD) also wrote about the mineral in his works "Naturalis Historia" and frequently made reference to Theophrastus' work "On Stones".

Pliny states;

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 different types of agate and the locations where they can be found.  Like Theophrastus he also makes reference to dendrachates (dendritic agate) which he says is "marked with small shrubs".  Dendritic agate is included with dendrites which are crystals with a branching formation that can often resemble plants or other organic matter.  Pliny talks about 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 and agates can also be found in Cyprus.  He believed stones with uniform colour would render athletes invincible and went on to say the variety with the highest value had the transparency of glass.

 

 

large oval shaped crazy lace agate stone on a black background


Crazy Lace Agate | Natural History Museum Los Angeles 

 

 

What Exactly is Agate?


Agate is a translucent variety of the mineral chalcedony. made up almost entirely of silicon dioxide otherwise known as quartz.  It often occurs in shades of black, white and grey but can also be exceptionally colourful.  Any colour and banding that's present is the result of trace impurities of other minerals.

The quartz crystals in agate are colourless in their purest form but turn red or brown with the presence of iron, pink with manganese or dark green with chromium. Other geological factors including light, heat or irradiation also play a part in defining colour which is the case with most rocks and minerals.

Some would argue that to be classified as a "true" agate the stone must exhibit banding but there are several varieties which do not exhibit this characteristic at all. Over the years there have been endless discussions regarding the accuracy of the names of certain minerals and it's safe to say that some are highly likely from a geological perspective, to be incorrect or at least slightly misleading. Having read and written about rocks and minerals for many years, my advice is not to look too deeply into a name.

Moss and dendritic are two varieties that don't exhibit any visible banding but feature curious inclusions known as dendrites. Often mistaken for organic matter, dendrites are crystal formations similar to those seen on glass during winter and are caused primarily by impurities of iron and manganese. Dendrite comes from the Greek word for tree and refers to a "branching form".  Rocks and minerals that feature dendritic inclusions are sometimes mistaken for fossils.

 

 

dendritic inclusions in a beige coloured opaque piece of rock


Dendritic Inclusions

 

 

Although frequently found within igneous rocks agate can also occur in other rock types as well.  Its formation begins when ground water containing silica (sand or quartz) that has entered the rock through holes dries up leaving behind a residue of natural minerals. As the process repeats multiple layers of minerals build up within the rock following the outline of the cavity and once full, the entire mass begins to crystallise. Quartz crystals often grow in space that's left unfilled and when their tips point towards a void, the structure becomes known as a geode


The cavity containing the crystallised minerals is known as a nodule.  Depending on the rock type it can be incredibly resilient to weathering and will often survive long after the host rock has eroded. The agate formation will remain securely locked away until the nodule is found and broken open.

On Mohs scale of mineral hardness agate grades 7 so it's quite hard which makes it ideal for use as a lapidary material.

 

 

large agate stones on display in a museum display cabinet


Smithsonian Museum Washington D.C. Photo; Stone Mania

 

 

Different Agate Stones


Although often characterised by concentric, curved or angular banding agate can also exhibit other curious and often intricate patterns.  Hundreds of different varieties can be found around the world and many have their own unique name.  These often indicate the locality where the stone is found or a specific colour or characteristic.

Fortification agate is a generic name for varieties 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 other reinforcement built to strengthen a building against attack.  Blue lace and crazy lace are both types of fortification agate.  Botswana agate comes from Botswana, Brazil agate from Brazil, fire agate exhibits a distinctive flash of colour caused by inclusions of hematite and polka dot agate often features distinctive spots.  Agatized dinosaur bone despite its name is not an agate stone at all.  The word "agatized" is simply a description of a process that has taken place because the organic matter in the original cell structure of the dinosaur's bone was slowly replaced with the mineral agate.

 

 

colourful agate stone with distinctive banding. In a museum display cabinet


Natural History Museum Los Angeles

 

 

A Few More Facts


During biblical times agate was believed to have been the second stone in the third row of the high priest breastplate.  This religious garment adorned with twelve precious gemstones was worn by the Jewish high priest whilst presenting himself to God.

Although agate is often dependant on the presence of other minerals for its colour, it also occurs naturally in black, white and grey.  These colours are often not considered to be sufficiently appealing to the wider commercial market so stones often end up being dyed.

The practice of dying rocks and minerals is not new and can be traced back thousands of years. Agates have long been known to be porous with the ability to hold dyes particularly well and it can sometimes be difficult to determine whether a stone's colour is natural or dyed.

Despite everything that has been learnt about the formation of this mineral there's still a great deal that's not fully understood. Its formation from start to finish has never been studied in real time and it has not been possible to recreate it in a laboratory setting so much of what is known is supposition.

Agate is associated with the star sign Gemini and is the traditional birthstone for the month of May.

 

 

Article Photos


This first three and the last photograph in this article are courtesy of Stan Celestian on Flickr.  Clicking any image on this page will redirect to the original full sized photo.

 

 

Our Collection of Agate

 

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Read More

https://geology.com/gemstones/agate/ 
https://www.flickr.com/photos/bill_the_eggman/albums/72157638018796976

 

 

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