Agate

Agate is a huge and broad subject, covering many ‘varieties’, with various patterns, colours, and trade names.

To cover them all would take a huge amount of time, so I will give a little background, and a quick overview of some of the most popular types of Agate.

The most popular types of Agate does not necessarily mean the most common pieces, of course – simply the pieces that most are interested in purchasing.

Agates are amongst the most popular semi precious stones for jewellery, and decorative stones.

Common types of Agates include:

Agatised fossils are fossils which have been replaced by Agate or Chalcedony over the years. In some cases this can be spectacular – a favourite of mine are the agatised Corals that come from Indonesia.

Banded Agate is a bit of an unnecessary description; Agates are banded by definition. This term tends to be used when an Agate does not have any particularly distinguishing features and cannot be given another name.

Black Agate is quite a tricky one to spot. If the stone is completely opaque, with no banding evident, it may be Chalcedony, Obsidian, Tourmaline or another black stone.

If the stone is opaque with white banding, it is likely to be Onyx. If the stone is translucent, with dark stripes/banding, it is black Agate.

Blue Lace Agate is a distinct blue banded Agate whose main locale has been the Ysterputs mine in Namibia. Unfortunately, this mine is now dormant and the flow of material onto the market has slowed to a crawl, increasing the price of Blue Lace Agate dramatically.

Banded Carnelian is a bit of an unusual one. Carnelian refers to a orangey red to brown Chalcedony, which is not banded and therefore not an Agate. However, it is relatively common to find ‘banded Carnelian’, which is Carnelian banded with white Agate.

Condor Agate is a bright colourful material found in the mountains near San Rafael, Argentina. It often has bands of grey, red, pink, and orange. Unfortunately much less material is coming to market nowadays.

Crackled Agate would best be described as a treated stone. It is Agate which has been heated until it becomes spiderwebbed with small cracks, and after that it is typically highly dyed. I consider it a bit of a nuisance stone, as it is sometimes referred to as ‘crackled fire agate’. Fire Agate is a distinct and different entity so this is very misleading to the inexperienced.

Crazy Lace Agate is a banded form of Agate which is primarily white with layers of browns, greys, blacks, yellows, and reds. Most of the material available on the market today comes from Mexico, and can be spectacular.

Dendritic Agate is not strictly an Agate; it is not banded and therefore isn’t technically an Agate but a Chalcedony. It is a colourless, white, or grey tone with grey-green or grey inclusions of iron or manganese which are called dendrites.

The term ‘Tree Agate’ tends to refer to a very similar material, albeit with green dendritic inclusions that look more like trees.

Dyed Agate is a pretty wide description covering any and all dyed Agates, of which there are likely hundreds or thousands of varieties.

Fire Agate is an iridescent material, which isn’t strictly an Agate due to its banding. Due to inclusions of Goethite or Limonite, it produces a bright iridescent ‘fire’. Fire Agate can be extremely valuable when of a high grade – it could be mistaken for Ammolite in some cases.

Green Agate is often a dyed material, but there are some natural pieces, which are relatively rare. They were used as a cheap substitute for Emeralds.

Iris Agate is a finely banded, pale coloured Agate that provides a spectacular display of colour when lit from an angle that sends light through the bands. It is typically an orange-yellow colour until illuminated, at which point it takes on a rainbow iridescence. Much of the Iris Agate on the market comes from Indonesia.

Moss Agate is a form of Chalcedony with dendritic inclusions. It is not an Agate due to a lack of banding. Moss Agates are typically clear or white, with black or green dendritic inclusions which resemble moss. In some cases, other colours can exist, particularly in specimens from Indonesia.

Onyx is a form of Chalcedony with bands, however – it is not an Agate. Technically speaking Agate has curved banding, and Onyx has parallel banding. Onyx is most commonly black and white. Banded brown materials from Mexico and Pakistan are not Onyx, they are a form of banded Calcite that is being missold by many dealers.

Polyhedral Agate is an interesting type of Agate from Brazil, which has formed in a very unusual way. It is thought that the Agate has formed in the voids between other crystals, which have then dissolved away – this has left an unusually shaped natural Agate.

Sagenite Agate is an agate (or sometimes Chalcedony, this varies piece-by-piece) which contains acicular or needlelike growths of minerals, often including Rutile, Hematite, or other needle-like metals.

Sard is a material similar to Carnelian, a form of Chalcedony. The key different between the two is purely in colour – Carnelian is an orange-red colour, Sard is a darker red-brown colour.

Sardonyx is an odd one. It is technically a form of Onyx with a red-brown base colour, from Sard, mixed with layers of white Agate, or different tones such as Carnelian.

Stick Agate is a little tricky to define specifically; it may be banded and it may not be, so it can be an Agate and not an Agate. The most accurate definition would probably be something like “Chalcedony with tube Agate inclusions”.

Tree Agate refers to a very similar material to Dendritic Agate, albeit with green dendritic inclusions that look more like trees.

Tube Agate is an Agate containing visible flow channels or small ‘tubes’, which are either a result of how the mineral formed or the result of inclusions.

Turritella Agate is a fossiliferous form of Chalcedony; it is not an Agate as it does not feature banding. As it turns out, the gastropods fossilised inside the stone are actually from the Pleurocereid family, not the Turritellid family – so both parts of the name are actually wrong. It should technically be called Elimia Chalcedony, or Chalcedony replaced Elimia.

It is found in the Green River Formation of Wyoming.

Zebra Agate is another material that is quite misleading. It is, in all likelihood, a form of Chalcedony or Quartzite, rather than an Agate. It appears to be mined in Utah, USA and should not be confused with a brown ‘Zebra stone’ mined in Australia.

In addition, there are several Agates named after their most prolific source.

Botswana Agate is a grey and white banded Agate from Botswana. It may also feature peach and pink colours.

Fairburn Agate is an Agate found around South Dakota and in parts of Nebraska. It shows strong colours including red, pink, orange, blue, yellow, brown and white – and the layers change direction quite often, making it distinct.

Lake Superior Agates are found along the banks of Lake Superior, between the USA and Canada. It is often predominantly red, due to iron found extensively around the area. It is extremely popular in America.

Patagonia Agate is an extremely unusual Agate from Patagonia, Argentina – it is also known as ‘Crater Agate’ or ‘Red Fox Agate’.

It is hard to determine whether this is technically an Agate, as it is formed in nodules with a very dark brown or black colour. The nodules have a very rough exterior, and when cut it is revealed to have a botryoidal red centre comprised of Jasper and Hematite.

Scottish Agates are, of course, Agates from Scotland. There are not nearly as many Agates in Scotland as many of the other countries which produce Agate, such as Brazil.

However, Scotland does still produce a wide range of Agates, some of which are of exceptional quality. They were very popular for jewellery in the UK during the reign of Queen Victoria.

In the 1870s there was an estimated 2000 people working Agates and metals in Scotland.

Agates were commercially mined at Burn Anne, but there is no longer any commercial mining of Scottish Agates except perhaps by individuals.