Agate Stone | Meaning and Properties

 

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

 

 

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 traced back to his works Theophrastus On Stones in which he compares the hardness of agate to onyx and talks about how one variety differs from the other because of "its irregular and uncertain manner of spots, clouds and variegations".  He says that although predominantly grey, different colours are 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 agates 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 and achates as it was known at that time, came from 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" and those featuring 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 was highly valued by many ancient cultures including the Sumerians, Egyptians, Greeks and Romans and was 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 agate 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 varieties of agate and the locations where they can 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 varieties of agate from India and their distinctive markings and says the sight of some stones 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 those stones 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 polished 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. that's made up almost entirely of silicon dioxide otherwise known as quartz.  Although stones often occurs in shades of black, white and grey they can also be exceptionally colourful.  Any colour and banding that is present is the result of trace impurities of other minerals.  The quartz crystals 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 and heat also play a part in defining colour which is the case with most rocks and minerals.

Some would argue that in order to be classified as a "true" agate a 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, the best advice on this matter is to not look too deeply into a name.

Moss and dendritic are two varieties of agate 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. The word 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 layers of minerals build up which follow the outline of the cavity and once full, the entire mass begins to crystallise. Quartz crystals often grow in any space that's left unfilled and where their tips point towards a void the structure is known as a geode.  The cavity containing the crystallised minerals is known as a nodule and depending on the rock type can be incredibly resilient to weathering so 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 a hard stone which makes it ideal for use as a lapidary material.

 

 large agate minerals on display in a museum display cabinet


Smithsonian Museum Washington D.C. Photo; Stone Mania

 

 

Varieties of Agate


Although often characterised by concentric, curved or angular banding agates 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.  Names often indicate the locality where the stone is found found or a particular colour or characteristic.  Fortification agate for example is a generic name for stones whose bands are arranged at sharp angles and came about because their shape was believed to resemble the lines of a fortress.  A fortification is a defensive wall or other reinforcement built to strengthen a building against attack.  The lace agates which include blue lace and crazy lace are both varieties 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 polkadot agate sometimes features dots but not always.  Agatized dinosaur bone despite its name is not an agate at all.  The word "agatized" actually describes the process which has taken place because the organic matter in the original cell structure of the dinosaur's bone was slowly replaced by agate.

 

 

colourful agate mineral 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 precious gemstones was worn by the Jewish high priest.

Whilst many types of agate are dependent on the presence of other minerals for their colour, stones can also occur naturally in black, white and grey.  These colours are not considered by some to be sufficiently appealing to the wider commercial market so often end up being dyed.  The practice of dying rocks and minerals can be traced back thousands of years. Agate has long been known to be a porous material with the ability to hold dyes particularly well and it can sometimes be difficult to tell whether the colour of a stone is natural.

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

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

Mineral photos courtesy of Stan Celestian - Flickr.  Click the images to see the original photographs.

 

 

Our Collection of Agate

 

clickable shopping trolley

 

 

Further Reading

Geology.com
A Collection of Agate