Agate Stone – Characteristics, Varieties, Colour and Cutting

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Agate has always been highly valued, since ancient times, by Egyptians and Sumerians. Various beliefs believe that it has the power to energise the wearer and promote healing.

Like all other gems, various kinds of power and mystical properties are attributed to it. However, it is a gem notable for its multicoloured bands or nuances of the same colour.

In jewellery, it provides a versatile application in various types of designs.

Agate belongs to the quartz group, it is a banded chalcedony.

Agate Stone
Agate Stone

.Its name is believed to come from the word achates, which refers to the Achates River, now called Dirillo, in Sicily – Italy, where this mineral was first found.

Agate is characterised by the variety of colours, usually arranged in parallel bands. When they are cut transversely, they display a succession of parallel lines, with extremely fascinating patterns of colours and nuances.

These patterns make agate a unique and original stone.

It has long been used for decorative pieces, ornaments and utilities such as ashtrays, cutlery handles and soap dishes.

In old buildings, it is not uncommon to find agate door handles.

The history of agate is closely linked to the German town of Idar-Oberstein. Agates and jasper have been found there.

This region evolved as an important gemstone centre.

It is considered the largest centre for cutting and polishing agates 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 deposits of agate were discovered in Rio Grande do Sul, here in Brazil, by immigrants from Idar-Oberstein.

Immediately, the stones were sent to Idar-Oberstein for cutting and polishing.

Pedra Ágada Bruta
Pedra Ágada Bruta

.As an important centre of cutting and polishing, its craftsmen execute true works of art. Today, the most important quarries, apart from the Brazilian ones, are in northern Uruguay.

Agates

Gemological characteristics

  • Mineral class: quartz.
  • Crystalline 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.
  • Gloss: greasy to vitreous.
  • Fluorescence: varies according to the nature of the layer.
  • Fracture: conchoidal, sometimes granular.

The mechanism of the natural production of agate is not yet well understood, but it is believed that there is a filling, in cycles, of the lacunae of volcanic rocks, by fluids rich in silica or silicon dioxide.

This filling may be partial or total, from the edge to the centre, through micro-cracks or seepage channels.

The pattern and width of the zones depends on the concentration of silica in the fluids, the temperature, pressure, and timing of the flows of these fluids.

There is also the theory that the fluid silica droplets cool simultaneously with the rock in the lava, forming crystallisation in zones from the outside.

In addition to mines in Brazil and Uruguay, agate can be found in Australia, China, India, Russia, Egypt, the USA, Mexico and elsewhere.

The deposits where agates are extracted are usually very large, providing this gem with an affordable price.

However, when it has a pattern of fine, well-delineated bands and a strong natural colouration, its value naturally increases.

These are more difficult to find.

Varieties

Each variety and form of agate is given a name. Some are named for specific geographical localities or for specific patterns and colours, such as fire agate and eye agate.

Some species of chalcedony are named agate, but are not true scientifically because they do not have the bands or stripes.

The most common are: dendritic agate, landscape agate, mosquito stone and mossy agate.

Despite this, they are traditionally called agate and recognised as such by dealers and collectors. Like some types of carnelian, sardonyx and onyx, which have the bands, they are also called and commercially categorised as agates.

Often the same agate specimen can be classified under two or more trade names.

Below you will see some types of agate.

1. banded agate

Banded Agate Stone
Banded Agate Stone

Banded or striped agate – have bands of different colours and rounded layers with various thicknesses, often contain white quartz crystals.

2. Dendritic agate

Dendritic Agate Stone
Dendritic Agate Stone

Dendritic agate – colourless or whitish-grey translucent chalcedony that shows tree and root-like designs due to the presence of iron and manganese ions as well as trapped debris such as sand, ash or mud.

3. Umbu agate

Agate umbu stone
Agate umbu stone

Ágata umbu – the most commercialised and sought after stone from the region of Salto do Jacuí – RS, one of the largest Brazilian exporters.

It has a bluish-grey colouration and makes it possible, through dyeing, to obtain homogeneous colours.

4. Carnelian agate

Carnelian agate
Carnelian agate

Carnelian – most carnelian today is agate dyed by dissolving iron nitrate.

Its colour ranges from orange to brownish red, it is translucent and is mainly found in India and Uruguay.

5. Eye agate

Eye Agate Stone
Eye Agate Stone

Eye agate – has an annular design with a dot in the centre.

6. Mossy agate

Mossy agate
Mossy agate

Mossy agate – translucent, colourless chalcedony that has moss-like designs that are green or brown in colour.

7. Blue loop agate

Blue bow agate stone
Blue bow agate stone

Blue loop agate – light blue bands in a wavy loop shape.

8. Landscape agate

Landscape agate stone
Landscape agate stone

Landscape agate – is a dendritic agate whose dendrites look like a landscape.

9. Geode agate

Geode agate stone
Geode agate stone

Agate geode – has a layer of agate surrounding a cavity, often lined with a layer of small quartz crystals.

10. Fire agate

Fire agate
Fire agate

Fire agate – a chalcedony that is iridescent, it has a play of colour or “fire look” similar to opal.

11. Laguna agate

Ágata laguna
Ágata laguna

Mexican agate laguna – features bands with very dense colours and several “eyes”.

12. Agata montana

Ágata montana
Ágata montana

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
Walled agate

Walled agate – has designs like ramparts.

Agatised organic materials

1. turritella agate

Agate turritella stone
Agate turritella stone

Turritella agate – is formed from fossils of Turritella (marine gastropod) shells silicified in a Chalcedony base.

2. Agatised coral

Agatised coral stone
Agatised coral stone

Agatised coral – forms from old coral in the same way as agatised wood, it can display a variety of colours.

3. Agatised wood

Agatised Wood Stone
Agatised Wood Stone

Agatised wood – occurs when pieces of wood buried for millions of years in volcanic mud undergo a petrifaction process where the carbon and cellulose of wood have been replaced by silica.

Lapidation and Application

Agates can be found in large sizes and in a wide variety of shapes.

Jewellery with agate stone
Jewellery with agate stone

.They are mostly cut into cabochons to enhance their beauty. They are also found in slab forms with simple cuts, which are 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.

For this reason, it is considered a versatile gemstone, in addition to the possibility of dyeing, which gives it several colours and shades. It has high durability due to its hardness.

Colour and Dyeing

German agates were known and appreciated worldwide for their colours that ranged from a delicate red, pink to brown, separated by luminous grey middle layers.

Agate Colour and Dyeing
Agate Colour and Dyeing

Today, those found in other deposits, reveal shades of grey to bluish grey, some, especially the Brazilian ones, have shades of white, black, brown, red, yellow, among others.

Around 40 per cent of Brazilian agates are dyed and in other countries the figure is over 50 per cent.

As it is porous, it easily accepts the dyeing processes, in addition, it is resistant to heat and acids. Beautiful colours such as pink, purple, green and blue can be obtained.

It is estimated that at least 90% of the agates sold in the world are dyed.

As a consequence of the demands of foreign lapidaries, the extraction of agate was 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 dyeing is slower than heated dyeing.

As the degree of porosity varies in agates, some absorb more of the dye, this provides a greater contrast between colours.

This solution does not have great penetration in the gem, so the dyeing is done after the piece is cut and roughened and before polishing, as it clogs the pores, making 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. With organic dyes (used, for example, to obtain pink or green colour), the colour will be weakened over time.

Care

Agates can be cleaned with warm water, mild soap and a soft brush. Prolonged exposure to extreme heat should be avoided.

Store agate jewellery in boxes lined with velvet fabric. Avoid contact with other stones to prevent scratches or fractures.

Even though it is a durable and resistant gemstone, it is not recommended to use chemical products.

Avoid exposing your agate to light for a long time because, depending on the dyeing process it has undergone, its colour may be weakened.

Agate Stone – Characteristics, Varieties, Colour and Cutting

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