Agate (Many types)

Jennifer Lince
Sep 30
7 min read

*Image: Crazy Lace Agate, Tennis ball for size*

A Siliceous gemstone and one of the first materials known to man, the name “agate” is related to the “Achates” or “Acate” which is the name of the Dirillo River in Sicily. Historians believe this to be where Agate was first discovered by Theophrastus around 371-287 BCE.

Agate is a gemstone that forms within cavities of other rocks that are in acidic to neutral environments. Rock voids that have accumulated silica-rich groundwater are where agate can be found formed, mostly within the voids of volcanic igneous rocks and plutonic igneous rocks. The bands are a result of the levels of the groundwater over long periods of time as the stone forms.

Not categorizable as any one of the three geological rock types (sedimentary, metamorphic or igneous) agate is a type of chalcedony which is simply a generalized name for a microcrystalline quartz and moganite mineraloid. Both quartz and moganite have a silicon dioxide formula but differ in the crystal structure.

A geode is not the same as an agate crystal, geodes form a series of protruding quartz crystals along cavity walls, whereas agate types develop chalcedony that will completely occupy the void space and, in some cases, crystallize with or within a geode.

The physical properties of Agate (a mineraloid rather than a mineral) are gathered by the same methods as would be used to gather information on minerals.

Metaphysical Properties

According to legend, agate is used to make the wearer agreeable and persuasive, to cure insomnia and give its owner pleasant dreams.

Different types of agate

Dendritic Agate

This is where inclusions of iron, manganese or other metallic oxides form in fern-like appearances called ‘dendrites’, creating bold patterns within the stone.

Picture Agate

This form of Agate has inclusions that have formed within the stone in a way that forms a natural, life-like, picturesque image that often looks like a landscape.

Iris Agate

This is a phenomenon rather than a type of agate and occurs when agate is very thinly sliced. As Agate is a semitransparent to translucent member of the chalcedony species, iris agate exhibits a beautiful iridescent rainbow-like range of colours and is highly prized by collectors.

Lace Agate

Blue Lace Agate comes from the Republic of Namibia and Kenya and is well known for its baby blue and white wavy bands that are reminiscent of lace.

Sagenitic Agate

Loosely translating to “large net”, some of the best-known sagenitic agates are Crazy Lace Agate and Trent Agate and have a network of needle-like minerals, embedded secondary crystals or crystal replacements.

Crazy Lace Agate is also known as Mexican Agate and is a banded chalcedony that is infused with iron and aluminium. It is most often brightly coloured, however, the white variety of Crazy Lace Agate is incredibly attractive and highly sought after.

Thunder Egg

This is similar to a filled geode, however, it is formed within rhyolitic volcanic ash layers as rough spheres (most are around the size of a baseball) that can range from under an inch to over a meter wide and usually contain centres made of chalcedony. This chalcedony is often fractured with agate, jasper or opal and often has quartz and gypsum crystal along with other mineral growths and inclusions.

Turritella

This form of Agate has large fossil snails included within it.

Carnelian

This is just a variation of agate in a different colour and has no ‘sub-category’ to fall under but is identified by its vibrant range of fire-orange reds to brown-reds and has a very dull and waxy lustre.

Fire agate

This is an opaque variety of chalcedony that has a colourful iridescence that is caused by layers of silica and iron oxides that interfere with the passage of light, using diffraction to cause colourful metallic hues.

Black Onyx Agate

This is where Agate and Onyx are both layered chalcedony varieties that differ only in band form (where agate has curved bands and onyx has parallel bands) and can appear in nearly any colour. This stone is often found being sold where it is actually another form of agate that has been dyed.

Sardonyx Agate

This is where the coloured bands are Sard (shades of red) rather than black and black onyx is perhaps the most famous variety of but isn’t as common as onyx with coloured bands.

There is a lot of this in the market where the colours have been artificially added and are difficult to identify without looking at it from a scientific viewpoint.

Brazilian Agate

This is the most common that you will see and when you get pieces of agate that have been dyed to bright colours or cut into book ends etc. chances are, it is Brazilian Agate.

Polyhedroid

This type of Agate has flat sides and layers of condensed polygons can be seen inside it when it is cut open.

Botswana Agate

Comes in dark and light grey and pink shades and is highly valued.

Plume

The mineral inclusions in Plume Agate resemble billowy clouds or feathers and the best-known variations are Bloody Basin, Flame Plume and Graveyard Point Plume Agate.

Coyamito Agate Pseudomorph

This is a stone that has gone through the formational process where crystals grow in the gas cavity left in a volcanic andesite rock (or aragonite in the case of the Coyamito Agate). The agate then forms over the crystals and cavity until the cavity is filled (although more often than not, the nodule will leave a hollow portion).

Condor

Discovered by Luis de Santos in 1993, this variety of Agate is found in the mountains near San Rafael (Argentina), has colourful bands and patterns and is a popular stone among collections and jewellery designers.

Fairburn Agate

This is a rare form found in Fairburn, South Dakota and is considered to be the state Jewel of South Dakota. When it is split into 2 pieces, you can beautiful patterns of colours.

Holley Blue

This is often mistaken for Blue lace agate but it isn't always banded and comes as a translucent violet-blue colour.

Moss or Tube Agate

This type of agate is where inclusions that look like moss or interconnecting tubes are a part of the stone. The most well-known varieties are Golden Moss, Indian Green Moss, Horse Canyon and Rooster Tail Agate.

Moss Agate's green colour comes from Chlorite, while the reds and browns come from manganese oxide or the oxidation of iron hornblende.

Fortification

This type of Agate is most recognised as Botswana agate, Blue Lace Agate and Laguna Agate and has nearly parallel alternating bands of colour and is the most recognizable type of agate.

Lake Superior Agate

This is a type of agate that is stained by iron and found on the shores of Lake Superior. It was designated as the official state gemstone for Minnesota in 1969 thanks to its wide distribution of iron-rich bands that reflect the stone's geologic history in Minnesota.

Binghamite

This is a variation that is also called silkstone and cuyunite and is only found on the Cuyuna Range in Minnesota.

Patuxent River

The state gem of Maryland, it is only found in (you guessed it), Maryland, USA and is said to be composed of fragments of petrified bone that were filled with silican and clay over 100 million years ago.

Enhydro

Enhydro is literally, a pocket of water within a stone. Enhydro Agate is agate with water in its central cavity.

What makes the different types of Agate?

Different timeframes for crystallization and geologic activity, differing environmental components and different geochemistry, groundwater content and pH levels are what create the different types of Agate.

What is the rarest variety of agate?

The rarest agate is called the Plentitude stone and is considered to be the most valuable form of agate.

Stories/legends about them

In Greek mythology, Agate is associated with Mother Earth, the Goddess Gaia and also designates the Goddes of the Night, Nyx.

Romans associated agate with Aurore (or Aurora), the goddess of dawn and the mother of winds and it has also been linked to Bona Dea, the Goddess or Virtue and protector of Women.

The Sumerians used this stone by wearing it around the neck to conjure the curses of Goddess Lamashtu, threatening newborn children and in the Epic of Gilgamesh, it has been said that in the garden of the Gods, dewdrops were agates.

In Ancient China, they believed that it came from the brain of a fossilized horse and was also supposedly from the spilt blood that had recrystallized during the Era of the Yellow Emperor (2500-2600 BC).

Colours it can be found in

Agate can be found in a variety of colours and forms depending on the type of agate it is, where it is found and the minerals that have contributed to its formation.

The colour variations within each piece are due to the alternating silica composition, pH and chemical properties in the water at the time of crystallization.

Scientific Properties of Agate as a whole

Mohs hardness: 6-7

Formula: Sio2 (Silicon Dioxide)

Occurrence: Nodules

Fracture: Uneven

Is a variety of: Chalcedony

Cleavage: None

Crystallography: Hexagonal (trigonal), cryptocrystalline

Refractive Index: 1.530-1.543

Birefringence: up to 0.004

Dispersion: None

Luminescence: Fluorescence varies with bands, somewhat strong: yellow, blue-white