What Is Agate And How Do Agates Form? Simple Explanation

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Most people have definitely seen agate in their lifetime (either in real life or in a movie/on the Internet) while many others have heard of them and know that it’s a type of rock. Well, it’s actually a very popular rock that’s used in different industries for a variety of tools but it’s more famous for its use in jewelry and decorations. However, not many people know what it’s made of or how it’s made, which is what this article is all about. So what exactly is agate and how is it formed?

Agate is a type of semi-precious stone made of chalcedony and comes in a large range of colors. All agates feature bands of colors with only a few exceptions. Having bands of colors is one of the key characteristics that makes it a real agate. Agates most commonly form in cavities within volcanic rocks, when silica-rich water flows into these cavities and solidifies.

But there’s another very famous rock called jasper that’s made of chalcedony but is different from agate. On top of that, even though having bands of colors is one of the main defining features of an agate, there are some agates that don’t have them. Confused? Don’t worry it’s actually quite simple and the subtle differences are explained below.

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What is Agate?

Agate is a very old (takes 50 million years to form) and widespread rock that has bands of color on it. The bands may have different hues of one color (e.g. blue lace agate which has a light and dark shades of blue) or feature completely different colors on the same stone (e.g. iris agate which shows all the colors of the rainbow when correctly illuminated).

Another important characteristic that all agates have is translucency – which means they allow light to pass through them (especially near the edges).

Even though some agates don’t have bands and are classified as “agates”, they are not considered real agates.

So they are banded rocks that have a certain degree of transparency. But what are they made of? Agates are made of chalcedony, but that brings forward the second question – what’s chalcedony made of?

Chalcedony is made of a specific type of quartz and another element called moganite. The quartz that’s found inside chalcedony is called microcrystalline quartz because it’s made of tiny crystals that can only be seen with a microscope. There are actually many types of chalcedony besides agate which are discussed later on.

So to wrap up – agate is a special type of chalcedony that’s different from the others thanks to its two main features:

Translucency
Bands of color

How Does the Agate Form?

Agates form in igneous (specifically those of volcanic origin like basalt) or metamorphic rocks and are usually found as small masses (“nodules”) inside the cavities of these rocks.

What’s important is that agates don’t form at the same time as the rock – they form inside (or sometimes on) the rock long after the rock has formed.

When the lava cools down, different gases form bubbles inside these volcanic rocks. These bubbles then solidify and create small pockets inside the rock known as cavities (imagine Swiss cheese). Agates from inside these cavities.

When water that has a high concentration of silica (SiO₂– quartz) flows into these cavities, it changes to a semi-solid state of silica gel, which deposits on the walls of the cavity. The water eventually seeps out or evaporates and the silica gel crystallizes on the walls.

Over time, new layers of silica gel enter the cavity and build upon the previous layers.

This usually happens in two ways – either in concentric circles (starting from the walls and filling up towards the center) or horizontally (the first layer is at the bottom and each new layer builds up on top of the previous one).

Since each layer of silica gel comes from a different time period, they have different mineral compositions and impurities. This is the reason that agates are banded.

One of the most common structures is alternating layers of chalcedony and crystalline quartz. An interesting fact is that sometimes not enough silica gel enters the cavity and the agate doesn’t fill up completely – leaving a hollow core.

TIP: Many people are not sure whether Agate is a rock or a mineral. That’s why I wrote this article for you, where I will explain the difference between rocks, minerals, and crystals, check it out here:
Rock, Mineral, or Crystal? What’s the Difference?

What Are the Main Types of Agates?

Agates can come in a variety of natural colors like red, green, blue, yellow, brown, white, orange, pink, purple, black, and grey. They often feature combinations of the colors mentioned above in different bands.

But agates are classified into different types based on more than just their color. Here are some of the most famous types with descriptions:

Blue lace agate

Features bands of light blue often with darker hues in between.

Crazy lace agate

Features swirls, twisted bands, and circular patterns in a variety of colors, especially red, gold, brown, white, and cream.

Moss agate

Semi-transparent (if it’s not a large, thick piece) and features green inclusions that resemble moss. The green “moss” is actually hornblende. Although this is a type of chalcedony and doesn’t have bands, it’s still called an agate.

Dendritic agate

Also translucent with black or brownish inclusions in the shapes of trees or ferns. These are also not real agates because they don’t have bands and are actually another type of chalcedony.

Nevertheless, they’re called agates and are also known as “landscape agates” since they can resemble actual landscapes like forests, mountains, and trees. Those that look like real landscapes are very valuable and collectors are always on the lookout for them.

Fire agate

An expensive type of agate that displays iridescence (play of colors when viewed from different angles). The iridescence is the result of limonite or goethite inclusions.

Onyx

Though onyx is usually sold as a separate gemstone, it’s actually a type of chalcedony very similar to agate. The biggest difference is their bands – onyx has bands that are parallel to each other, whereas agate bands are curved.

The most common band colors for onyx are white and black, though you may come across onyx in a variety of other colors (e.g. red). It’s often passed off as a type of agate.

Sardonyx

Features parallel bands of brownish-red and sometimes white or even black. The name comes from the two rocks – “sard” and “onyx”.

Iris agate

Another iridescent agate has fine bands that display all the colors of the rainbow when it’s illuminated from behind at a correct angle.

To see the brilliant colors the agate needs to be cut into a thin slice and a source of light needs to be placed behind it (this means that it doesn’t work as jewelry).

Eye agate

Features bands of concentric circles that resemble an eye.

Agate geodes

These are spherical rocks that are cut (often split down the middle) to reveal the crystals inside. They often feature banded agate and quartz crystals as well as other minerals such as calcite or celestite. For it to be considered a geode, the core has to be hollow.

Thunderegg (or thunder rock) agates

Very similar to agate geodes but with slight differences. First off, they have a specific structure while “geode” is a general term for any round rock with a hollow core and crystal growth.

Secondly, they are usually solid and not hollow. They contain chalcedony as well as agate, opal, gypsum, or jasper. These elements can be present in the rock together or separately in different rocks.

Laguna agate

Found in Mexico, these are famous for their red/scarlet bands that are very close together (tight banding).

Turritella agate

A brownish, translucent agate that contains large fossils of snails.

Lake Superior agate

As the name suggests, this agate is found on the shores of Lake Superior in the United States. It has a reddish-brown color which is the result of the iron it contains and is the official gemstone of Minnesota.

Sagenite agate

Features unique golden-yellow, hair-like formations.

Fortification agate

Features concentric bands that outline the shape of the cavity.

TIP: Agate can be found almost anywhere on Earth, but is it a valuable rock? Find out the values for different types of agate in the article below:
Are Agates Valuable rocks? The True Worth of Agates

Are All Colors in Agate Real?

Unfortunately, the answer is no. Many agates (especially tumbled ones) are dyed to artificially enhance their color. This is done to make them more appealing to buyers and increase sales.

The stones that are dyed usually have a weak color to begin with, which means a lot of people wouldn’t be interested in them if they weren’t dyed. Agates are porous – making them very easy to dye.

Dyed agates usually have very strong and intense colors, making them easy to spot in some cases. Green, blue and red are the most common colors for dyed agates but there are also pink, brown, and black dyes.

Some people have no problem buying dyed agates since they are technically still agates while others refuse to buy them and argue that those are not their natural colors. A lot of agates from Brazil are dyed.

BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):

What Does Agate Look Like in Nature?

Identifying unpolished, rough agate outside can be difficult. They are often roundish but can take different shapes, and they may or may not have openings.

They are usually less than 3 inches in diameter. One key feature agates have is a waxy luster. When you look at an unpolished agate or hold it in your hand and feel it, it will remind you of candle wax.

They are hard rocks and often have a greyish color on the outside, though they may have bands of colors, which automatically gives them away as agates.

Experienced rock hounds search in areas that are known to have agates to make it easier to find them.

TIP: Agates look quite dull from the outside. It is when they are cut and polished that their intricate wavy pattern. Check out the complete guide on cutting and polishing agates in the article below:
How to Cut and Polish Agates? Follow These Simple Steps

How Can You Tell if a Rock is an Agate?

The main things to look for when identifying agates are:

Waxy luster – (explained above)
Weight – agates are dense stones, so if you hold them in your hand they should be heavier than it looks,
Banding – the key characteristic of agates. If it has bands of colors then it’s most likely an agate,
Check for conchoidal fractures – the most common type of fracture for agates as well as obsidian. These fractures have a curve-like appearance,
Translucence – if the stone has no opening you will need to use a hammer and chisel to break it open. Then you need to shine a light on the agate from behind and look for signs of transparency, especially near the edges. Agates always have some level of transparency (though it’s more noticeable when the agate is cut in a thin slice).

FAQ about Agate Rocks

Still did not find the answer to your answers about agate rocks? Find frequently asked questions in the section below:

How to Dye Agates?

There are numerous methods of dying agates, some of which include:

Boiling the stone in a solution containing dye (the heat expands the stone making it easier for the dye to enter),
Heating up the stone in a solution containing dye and then dropping it into a cold solution (the rapid temperature drop causes fractures which make it easier for the dye to penetrate different parts of the stone),
Boiling the stone in a bicarbonate solution than putting the stone in a different chemical solution (the chemical used depends on what color you want the rock to be).

To get a red agate an iron nitrate solution is used, blue agates require red or yellow prussiate of potassium, and getting a green agate requires nickel nitrate or chromic acid.

How Can I Tell a Dyed Agate From a Natural One?

Though it may be difficult in some cases (if the dye is very similar to its natural color), here are a few tips to make it easier to tell the difference:

Extremely unnatural color – if the color is very bright, neon-like, and not usually found in nature, then it’s likely to be a dye.
Accumulation of the dye – if you look closely and see areas that have darker coloring, this could be a sign that the dye accumulated in that area, making that part darker than the rest of the agate. Areas like these are usually fractures and pits.
Surface penetration – since the dye doesn’t go very deep if there are any scratches or chips on your agate look closely to see if the inside of the stone has the same color as the outside. If you break a dyed agate, the color inside will definitely be different from the color it has on the outside.

TIP: Agate occurs in a variety of colors and textures, the combination of which can satisfy any taste. At the same time, there are a lot of fakes imitating agate. Find out the difference between real and fake agate in the article below:
Real vs. Fake Agate: You Should Know These 7 Differences

What’s the Difference Between Jasper and Agate?

Both jasper and agate are 2 different types of chalcedony. One of their main differences is that agate is semi-transparent whilst jasper is opaque.

This means that if you hold a thin agate slice in front of a light source, you should be able to see a bit of light pass through the edges.

Jasper, on the other hand, doesn’t allow light to pass through it. This is because jasper contains other elements and isn’t made of just quartz or chalcedony. It often has iron particles that give it a reddish color. The translucency is more apparent when the rock is cut into thin slices.

Agates also feature bands of colors, whereas jasper doesn’t. Some “agates” don’t have bands but these are exceptions (e.g. moss agate, dendritic agate).

What’s the Difference Between Quartz and Agate?

Agates contain quartz but the quartz inside agates is not just ordinary quartz. It’s microcrystalline quartz (tiny crystals of quartz that can only be viewed with a microscope) and it’s a part of the chalcedony that makes up the agate (chalcedony also contains an element called moganite). Agates contain both chalcedony and pure quartz.

TIP: Do you know where to find agate? I wrote a useful guide with all the important locations with agates in the USA and also Europe and other countries around the world. Feel free to read it here:
Where Can I Find Agate Rocks? Best Places in the US & World