Agate has always been highly valued, since antiquity, by Egyptians and Sumerians. Several beliefs believe that it has the power to energize those who use it and promote healing.
Like all other gems, various types of power and mystical properties are attributed to it. However, it is a notable gem for its multicolored bands or nuances of the same color.
In jewelry, it provides a versatile application in various types of design.
Agate belongs to the quartz group, it 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 found for the first time.
Agate is characterized by the variety of colors, usually arranged in parallel bands. When they are cut crosswise, they exhibit a succession of parallel lines, with patterns of colors and extremely fascinating nuances.
These patterns make agate a unique and original stone.
For a long time, it was used to compose decorative pieces, ornaments and utilities such as ashtrays, cutlery handles and soap dishes.
In old buildings, it is not uncommon to find agate handles.
The history of agate is closely linked to the German city of Idar-Oberstein. In it, agates and jasper were found.
This region has evolved as an important center for gemstones.
It is considered the largest agate cutting and polishing center in the world. Until the beginning of the XNUMXth century, Idar-Oberstein had the most important deposits of agate, however, from that date, they are 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.
As an important center for cutting and polishing, its craftsmen execute true works of art. Today, the most important deposits, in addition to the Brazilian ones, are found in the north of Uruguay.
- 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.
- Color: Variable, banded or layered, gray, white, red, and brown.
- Luster: from greasy to vitreous.
- Fluorescence: varies according to the nature of the layer.
- Fracture: conchoid, sometimes granulated.
The mechanism of natural agate production is still not well understood, it is believed that there is a filling, in cycles, of gaps in volcanic rocks, by fluids rich in silica or silicon dioxide.
This filling can be partial or total, from the edge towards the center, through micro-cracks or infiltration channels.
The pattern and width of the zones depends on the silica concentration in the fluids, the temperature, pressure, and timing of the fluid flows.
There is also the theory that the drops of fluid silicon cool, simultaneously with the rock, in the lava, forming a crystallization in zones from its exterior.
In addition to the mines in Brazil and Uruguay, agate can be found in Australia, China, India, Russia, Egypt, USA, Mexico, among others.
The deposits where agates are extracted are generally very large, providing this gem with an affordable price.
However, when it has a pattern of fine, well-defined bands and strong natural coloring, its value naturally increases.
These are harder to find.
With each variety and form, agate has a name. Some by specific geographic locations or by patterns and colors, also specific, such as fire agate and eye agate.
Some species of chalcedony receive the name agate, but they are not scientifically true because they do not have the bands or bands.
The most common are: dendritic agate, landscape agate, mosquito stone and mossy agate.
Despite this, they are traditionally called agate and recognized as such by traders and collectors. Like some types of carnelian, sardonyx and onyx, which have bands, they are also called and classified commercially as agates.
Often the same specimen of agate can be classified with two or more commercial names.
Below you will see some types of agate.
1. Banded agate
Banded or striped agate – have bands of different colors and rounded layers with different thicknesses, often containing white quartz crystals.
2. Dendritic Agate
Dendritic Agate – Colorless or whitish gray translucent chalcedony that features tree-like designs and roots due to the presence of iron and manganese ions, as well as trapped debris such as sand, ash, or mud.
3. Umbu agate
Agata umbu – the most commercialized and sought after in the region of Salto do Jacuí – RS, one of the largest Brazilian exporters.
It has a bluish-grey color that makes it possible, through dyeing, to obtain homogeneous colors.
4. Carnelian Agate
Carnelians – Most carnelians today are agates dyed by a dissolution of iron nitrate.
Its color ranges from orange to brownish red, is translucent and is found mainly in India and Uruguay.
5. Eye agate
Eye agate – has an annular design with a dot in the center.
6. Mossy Agate
Mossy agate – translucent, colorless chalcedony, which features green or brown moss-like designs.
7. Blue lace agate
Blue lace agate – light blue streaks in a wavy bow shape.
8. Landscape agate
Landscape agate – is a dendritic agate whose dendrites resemble a landscape.
9. Agate geode
Agate geode – has a layer of agate that surrounds a cavity, often lined with a layer of small quartz crystals.
10. Fire Agate
Fire Agate – A chalcedony that is iridescent, has a play of colors or “fire aspect” similar to that of opal.
11. Lagoon agate
Mexican lagoon agate – presents bands with very dense colors and several “eyes”.
12. Montane Agate
Montana Agate – also known as mossy agate, comes from the state of Montana, USA, known for producing the most beautiful agates in the world.
13. Walled Agate
Walled agate – has designs like walls.
agatized organic materials
1. Turritella agate
Turritella Agate – is formed from Turritella shell fossils (marine gastropod) silicified on a Chalcedony base.
2. Agate coral
Agate coral – forms from ancient coral in the same way as agate wood, can display a variety of colors.
3. Agatized wood
Agatized wood – occurs when pieces of wood buried for millions of years in volcanic mud undergo a petrification process, where carbon and wood cellulose have been replaced by silica.
Cutting and Application
Agates can be found in large sizes and in a wide variety of shapes.
They are mostly cut into cabochons to enhance their beauty. They are also found in the form of plates with simple cuts, which are normally used for ornamental designs, such as cameos and other sculptures.
This gem can be used in any type of jewelry design, in earrings, rings, necklaces, pendants and bracelets.
Therefore, it is considered a versatile gem, in addition to the possibility of dyeing, which gives it different colors and tones. It has high durability due to its hardness.
Color and Dyeing
German agates were known and appreciated worldwide due to their colors that varied from a delicate red, pink to brown, separated by intermediate layers of luminous gray color.
Today, those found in other deposits, reveal shades of gray to bluish gray, some, mainly the Brazilian ones, show shades of white, black, brown, red, yellow, among others.
Around 40% of Brazilian agates are dyed and, in other countries, the numbers reach over 50%.
As it is porous, it easily accepts dyeing processes, in addition, it is resistant to heat and acids. One can get beautiful colors like pink, purple, green and blue.
It is estimated that at least 90% of the agates sold in the world are dyed.
As a result of the requirements of foreign cutters, agate extraction was concentrated on the banks of the Jacuí River - RS.
In the dyeing process, agate is placed in a solution of potassium ferrocyanide, chromic acid with ammonium chloride, sugar or iron perchloride with nitric acid and scrap iron.
Cold dyeing is slower than heat dyeing.
As the degree of porosity is variable in agates, some absorb more dye, which provides greater contrast between colors.
This solution does not have great penetration into the gem, so dyeing is done after the piece has been cut and ground and before polishing, as it clogs the pores, making it difficult for the dye to penetrate.
Dyeing does not change the price of agates, however, if the dyes used are inorganic, the color will be stable. As for organic dyes (used, for example, to obtain a pink or green color), the color will weaken over time.
Agates can be cleaned with warm water, mild soap and a soft brush. Prolonged exposure to extreme heat should be avoided.
Store agate jewelry in boxes lined with velvet fabric. Avoid contact with other stones to avoid scratches or fractures.
Even though it is a durable and resistant gem, the use of chemical products is not recommended.
Avoid your agate being exposed to light for a long time because, depending on the dyeing process it was subjected to, it may have a weakened color.