The Mineral Agate
Agate is one of the oldest of all minerals and references to this stone can be traced back more than two thousand years to the writings of Theophrastus the Ancient Greek philosopher (c.370-285 BC). A distinctive and often colourful mineral, 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. It's also believed to have been one of the stones in the high priests breastplate also known as Breastplate of Aaron. This biblical garment is said to have been adorned with precious gemstones each of which represented one of the twelve tribes of Israel.
Relatively hard and grading 7 on Mohs scale of mineral hardness, agate is a translucent variety of the mineral chalcedony. Although usually characterised by concentric, curved or angular banding, stones can also exhibit other curious and often intricate patterns. It's particularly popular as a lapidary material and carefully selected cabochons can make beautiful pieces of jewellery.
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 continually repeats itself 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 remaining space. The cavity which now contains the crystallised minerals is known as a nodule and depending on the rock type in which it's formed, 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.
Agate on display in the Smithsonian National Museum of Natural History. Photo by Stone Mania ©
Agate is made up almost entirely of silicon dioxide so any colour and banding that may be 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, pink with manganese and chromium causes them to turn 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 form 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 crystals, 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.
Hundreds of different agate varieties can be found around the world and many will have a unique name. The first part of this name will often indicate the locality where the stone is found or it may denote a particular colour or characteristic. The process of describing different varieties of agate can be traced back to the works of Theophrastus who in his treatise Theophrastus On Stones, compares the hardness of agate to onyx and talks about how one 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 and 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". The difference could be a colour, marking 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 was from the river in Sicily by the same name where the mineral was first discovered. He subsequently wrote that agate has since been "found to exist in almost every nation on Earth". Stones featuring patterns resembling trees and shrubs were called dendrachates (from the word dendrite) and other varieties were "idly named" according to the powers the stones were supposed to hold or even their similarity in colour to animal skin.
Today the name Fortification Agate is used as a generic term for stones whose bands are arranged at sharp angles and this name came about because the patterns were said to resemble the lines of a fortress. A fortification is a defensive wall or other reinforcement which is 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. Polkadot Agate sometimes features dots but not always and despite its name, agatized dinosaur bone is not actually an agate at all. This 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.
Although agate is dependent on the presence of other minerals for its colour it frequently occurs in black, white and grey. Many consider these colours not to be attractive enough for the wider commercial market so they 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. Long known to be porous with the ability to hold dyes well, it can sometimes be difficult to tell whether an agate has been dyed or not. At Stone Mania we never knowingly buy crystals, rocks or minerals for our collection which have been artificially coloured and by that we mean dyed.
Despite everything that has been learnt over time about how agate forms, there's still a great deal that is not fully understood. Its formation has never been studied from start to finish in real time and it has not been possible to recreate the stone 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.
For further reading on agate feel free to explore our carefully selected links.