Agate Stone | Meaning and Properties
The History of 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, was from the river in Sicily by the same name where it was first discovered. He 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."
This distinctive and often colourful mineral 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".
In Pliny's work he says;
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. He too 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.
Agate was also 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 during biblical times.
What Exactly is Agate?
Agate which is a translucent variety of the mineral chalcedony. is made up almost entirely of silicon dioxide so any colour and banding that's present will be the result of trace impurities of other minerals. Crystals which are colourless in their purest form turn red or brown with the presence of iron, with manganese they turn pink and chromium turns them dark green. 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 show some kind of 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 with some degree of certainty 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 agate are well known for their lack of banding and instead 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 which feature dendritic inclusions are sometimes mistaken for fossils.
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 this process repeats, layers of minerals begin to form which follow the outline of the cavity and once full the entire mass begins to crystallise. Quartz crystals usually grow in any space that's left unfilled. The cavity which contains the crystallised minerals is known as a nodule and depending on the type of rock, it 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. If quartz crystals have grown and their tips point towards a void, the structure is known as a geode.
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.
Smithsonian National Museum of Natural History. Photo by 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 will have their own unique name. The first part will often indicate the locality where the stone is found or may denote a particular colour or characteristic. Fortification agate is a generic term for stones whose bands are arranged at sharp angles and the name came about because the patterns were said 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 Agate are both varieties of fortification agate whose patterns bare a resemblance to lace, Botswana agate comes from Africa, Brazil agate from South America and fire agate exhibits a distinctive flash of colour caused by inclusions of hematite. The variety known as polkadot agate sometimes features dots but not always and despite its name, agatized dinosaur bone is not actually an agate at all. This relatively rare material forms when organic matter is replaced with minerals during a process known as permineralization and although the minerals present may be calcite, common opal or iron pyrite, chalcedony tends to be the norm. The word "agatized" describes the process which has taken place because the organic matter has slowly been replaced by agate.
Natural History Museum Los Angeles. Photo Courtesy of Stan Celestian - Flickr
A Few More Facts
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 to be sufficiently appealing to the wider commercial market so often end up being dyed. The practice of dying rocks and minerals is not new and its history can be traced back thousands of years. Agate has long been known to be a porous stone with the ability to hold dyes really well and it can sometimes be difficult to tell whether a stone has been dyed or not.
Despite everything that has been learnt over time about how this mineral forms, there's still a great deal about it that's not fully understood. Its formation has never been studied from start to finish in real time and it has not been possible to recreate it in a laboratory setting so most of what is known about its formation is supposition.
Agate is associated with the star sign Gemini and it's the traditional birthstone for the month of May. It's used extensively in alternative therapies such as crystal healing and whilst its benefits and uses vary widely depending on the reference that you read, we tend to use The Crystal Bible written by Judy Hall as our main reference.
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