Agate has always been prized, from ancient times by the Egyptians and Sumerians. According to various beliefs, it has the power to energise the wearer and promote healing.
Like all gemstones, it is attributed with various powers and mystical properties. However, it is a gemstone that is characterised by its multicoloured bands or nuances of the same colour.
In jewellery it can be used in a variety of designs.
Agate belongs to the quartz group and is a banded chalcedony.
It is believed that its name comes from the word achates, which refers to the river Achates, now called Dirillo, in Sicily – Italy, where this mineral was first found.
Agate is characterised by a variety of colours, usually arranged in parallel bands. When cut crosswise, they show a series of parallel lines with extremely fascinating patterns of colour and nuance.
These patterns make agate a unique and original stone.
It has long been used for decorative pieces, ornaments and utensils such as ashtrays, cutlery handles and soap dishes.
It is not uncommon to find agate door handles in old buildings.
The history of agate is closely linked to the German town of Idar-Oberstein. Agate and jasper have been found there.
This region has become an important centre for precious stones.
It is considered to be the largest agate cutting and polishing centre in the world. Until the beginning of the 19th century Idar-Oberstein had the most important agate deposits, but since then they have been exhausted.
Around 1800, huge agate deposits were discovered in Rio Grande do Sul, here in Brazil, by immigrants from Idar-Oberstein.
The stones were immediately sent back to Idar-Oberstein for cutting and polishing.
An important centre for cutting and polishing, its artisans produce true works of art. Today, the most important deposits, apart from those in Brazil, are found in northern Uruguay.
Agates
Gemological characteristics
- Mineral Class: quartz.
- Crystal system: trigonal; microcrystalline aggregates.
- Chemical formula: SiO 2
- Hardness: 6 1/2 to 7 Mohs
- Density: 2.60 – 2.64
- Transparency: translucent, opaque.
- Colour: variable, in bands or layers, grey, white, red and brown.
- Lustre: from oily to glassy.
- Fluorescence: Varies according to the type of layer.
- Fracture: conchoidal, sometimes granular.
The mechanism for the natural formation of agate is still not well understood, but it is believed that there is a cyclic filling of the gaps in volcanic rocks by fluids rich in silica or silicon dioxide.
This filling can be partial or complete, from the edge to the centre, through micro-cracks or infiltration channels.
The pattern and width of the zones depend on the concentration of silica in the fluids, the temperature, pressure and time of the fluid flows.
There is also a theory that the liquid silica droplets cool simultaneously with the rock in the lava, forming crystallisation zones from the outside.
Apart from mines in Brazil and Uruguay, agate can also be found in Australia, China, India, Russia, Egypt, the USA and Mexico.
The deposits from which agate is mined are generally very large, making this gemstone affordable.
However, if it has a pattern of fine, well-defined bands and a strong natural colour, its value naturally increases.
These are harder to find.
Varieties
Agate has a name for each variety and form. Some are named after specific geographical locations or specific patterns and colours, such as fire agate and eye agate.
Some types of chalcedony are called agate, but this is scientifically incorrect because they don’t have the bands or stripes.
The most common are: dendritic agate, landscape agate, mossy agate and mossy agate.
Despite this, they are traditionally called agate and are recognised as such by dealers and collectors. Like some types of carnelian, sardonyx and onyx, which are banded, they are also commercially called and classified as agates.
Often the same agate specimen can be classified under two or more commercial names.
Below you can see some types of agate.
1. banded agate
Banded or striped agate – has bands of different colours and rounded layers of varying thickness, often containing white quartz crystals.
2. Dendritic Agate
Dendritic agate – colourless or whitish-grey translucent chalcedony that shows tree and root patterns due to the presence of iron and manganese ions and trapped debris such as sand, ash or mud.
3. Umbu Agate
Umbu agate – the most commercialised and sought after stone from the Salto do Jacuí region of Rio Grande do Sul, one of Brazil’s largest exporters.
It has a bluish-grey colour, which makes it possible to obtain homogeneous colours through dyeing.
4. Carnelian Agate
Carnelian – The majority of today’s carnelians are agates coloured by dissolving iron nitrate.
Its colour ranges from orange to brownish red, it is translucent and is found mainly in India and Uruguay.
5. Eye Agate
Eye agate – has a ring pattern with a dot in the centre.
6. Mossy Agate
Mossy agate – translucent, colourless chalcedony with green or brown mossy patterns.
7. Blue bow agate
Blue loop agate – blue bands of light in a wavy loop shape.
8. Landscape Agate
Landscape agate – is a dendritic agate, the dendrites of which resemble a landscape.
9. Geode Agate
Geode agate – has a layer of agate surrounding a cavity, often coated with a layer of small quartz crystals.
10. Fire Agate
Fire agate – an iridescent chalcedony with a play of colour or “fiery look” similar to opal.
11. Laguna Agate
Mexican Laguna Agate – has very dense bands of colour and several “eyes”.
12. Mountain Agate
Montana agate – also known as moss agate, comes from the state of Montana, USA, known for producing the most beautiful agates in the world.
13. Walled Agate
Walled agate – has patterns like ramparts.
Agate organic materials
1. turritella agate
Turritella agate – formed from fossils of Turritella (marine snail) shells silicified on a chalcedony base.
2. Agatised coral
Agatised coral – formed from old coral in the same way as agatised wood, it can have a variety of colours.
3. Agated wood
Agatised wood – occurs when pieces of wood buried in volcanic mud for millions of years undergo a petrification process in which the carbon and cellulose of the wood are replaced by silica.
Cutting and use
Agates can be found in large sizes and a wide variety of shapes.
Most are cut into cabochons to enhance their beauty. They are also found in the form of slabs with simple cuts, usually used for ornamental designs such as cameos and other sculptures.
This gemstone can be used in any type of jewellery design, in earrings, rings, necklaces, pendants and bracelets.
It is therefore considered to be a versatile gemstone, as well as being dyeable, which gives it different colours and shades. Its hardness makes it very durable.
Colour and staining
German agates were known and appreciated all over the world for their colours, which ranged from soft red to pink and brown, separated by bright grey mid-layers.
Today those found in other quarries show shades of grey to bluish grey, while some, especially those from Brazil, have shades of white, black, brown, red, yellow and others.
About 40 per cent of Brazilian agate is coloured and in other countries the figure is over 50 per cent.
Because it is porous, it easily accepts dyeing processes and is resistant to heat and acids. Beautiful colours such as pink, purple, green and blue can be achieved.
It is estimated that at least 90% of the agate sold in the world is coloured.
As a result of the demands of foreign cutters, the extraction of agate has concentrated on the banks of the Rio Jacuí – RS.
In the dyeing process, the agate is placed in a solution of potassium ferrocyanide, chromic acid with ammonium chloride, sugar or iron perchloride with nitric acid and iron scrap.
Cold staining is slower than hot staining.
As the porosity of agates varies, some will absorb more of the dye, giving a greater contrast between the colours.
This solution doesn’t penetrate very well into the gemstone, which is why dyeing is done after the piece has been cut and roughened and before polishing, as it blocks the pores and makes it difficult for the dye to penetrate.
Dyeing does not change the price of agates, but if the dyes used are inorganic, the colour will be stable. If organic dyes are used (e.g. to obtain a pink or green colour) the colour will fade over time.
Cleaning
Agates can be cleaned with warm water, neutral soap and a soft brush. Prolonged exposure to extreme heat should be avoided.
Store agate jewellery in boxes lined with velvet. Avoid contact with other stones to prevent scratches or breakage.
Although it is a durable and resistant gemstone, the use of chemical products is not recommended.
Avoid exposing your agate to light for too long, as the colour may fade depending on the colouring process it has undergone.
