Agate vs Chert: What Are They, And What's The Difference?

Cyrptocrystallines are rock-like substances made up of crystals, but crystals so minute that only microscopes reveal that crystalline nature.

Chalcedony is cryptocrystalline silica, which is silicon dioxide in mineral form.

Agates and cherts are both chalcedonies.

This article will describe both types of stones, discuss their similarities and differences, and describe methods to tell them apart.

Agate vs Chert (EXPLAINED)

Agate comes as a rock-like substance that primarily comprises quartz and chalcedony.

It can also come in a variety of colors.

Agate forms in volcanic or metamorphic rocks (rocks made from other types of rock due to heat and/or pressure).

Types Of Agate

Lace agate: this agate has lace-like patterns as swirls, eyes and bands.

Blue lace is an extremely hard agate that occurs in Africa.

Crazy lace, which is very common in Mexico, has brightly colored intricate patterns.

Moss agate: this agate is named for its mossy pattern.

It has a greenish coloration that is not caused by vegetation but by chalcedony combining with oxidized iron in dark amphibole (hornblende).

Turritella: forms from fossilised Turritellas shells. Turritella agate is also called ‘Elimia Tenera’.

Coldwater agates: these are not formed by volcanic processes.

This agate occurs within dolomite and limestone layers which are of mainly marine origin.

Coldwater agates are not as colourful as other agates.

An example of a cold water agate is Lake Michigan cloud agate.

Greek agate: refers to agate, ranging from pale white to dark tan, found on the island of Sicily.

Brazilian agate: occurs as large geodes in layered nodules.

The nodules are interwoven in white and grey layers but with an overall brownish tone.

Polyhedroid agate: these occur in flat shapes, which are polyhedrons.

Polyhedroid agate often displays layered concentric polygons when dissected.

(we often find agates in the Pacific NW in or near water)

A Brief Description Of Chert

Chert is a chalcedony occurring as a hard sedimentary rock.

It is commonly of organic origin, but it can also occur as an inorganic chemical precipitate found inside petrified wood.

Chert appears as concretionary masses or nodules or as well as in layered deposits.

Concretionary mass: these are hard, compact masses of matter.

The mass forms from the precipitation and encapsulation of cemented minerals interspersed within the cavities between particles of the encapsulating medium.

Concretionary masses are found in soil or sedimentary rocks.

They are usually spherical or oval; however, they can also take on irregular forms.

Nodule: in sedimentology or geology, is a small, irregularly spherical knot, lump or mass of mineral or a mineral aggregate.

Typically, nodules occur in contrast compositions.

As a nodule, chert may occur as a contrasting mass in limestone.

Types of Chert

There are many types of chert.

When classified by their visible, physical, and microscopic characteristics, we can identify these common varieties:

Flint: microcrystalline, compact quartz.

Flint was once the name of chert encountered in chalk and marly (a calcium carbonate-clay mixture) limestone formations.

Flint occurs when silica replaces calcium carbonate in limestone.

Geologists call any chert between dark grey and black “flint”.

In contrast, non-geologists usually distinguish between chert and flint by quality.

“Common” chert: the type of chert that occurs in limestone through the addition of silica and calcium carbonate.

This chert is less desirable than flint for making bladed instruments or gemstones.

Jasper: a type of chert that forms primary deposits. It is found inside or connected to magmatic formations.

Radiolarite: a type of chert with radiolarian (unicellular protists) microfossils and forms as a primary deposit.

Agate: banded chalcedony, successive layers of which differ in colour.

But wait! Is agate a chalcedony, or is chalcedony agate? Well, all cars are vehicles, but not all vehicles are cars.

All agate is chalcedony, but not all chalcedony is agate.

Onyx: banded agate that has layers that often lie in parallel white and black strata.

Novaculite: an extremely dense, uniform, fine-grained chert that is pure white.

Novaculite is abundant in mid-Paleozoic rocks in Oklahoma, Arkansas, and Texas.

Porcelanite: a siliceous, fine-grained rock. Its texture and fracture behaviour is like that of unvitrified porcelain.

Tripolitic chert: a brittle, light-coloured, siliceous, porous sedimentary rock.

It forms because of the decalcification of siliceous limestone due to weathering.

Tripolitic chert is also known as tripoli’.

Siliceous sinter: a porous, light-coloured, low-density siliceous rock.

The waters of geysers and hot springs deposit it.

Mozarkite: is a multicoloured Ordovician Chert. It easily withstands high polish.

Agate vs Chert: Similarities

General similarities

Several commonalities exist between agate and chert.

This is because both belong to the same family of stones.

Since all chalcedonies have either a semitransparent or waxy lustre, so does agate and chert.

Both stones come in the many colors that chalcedonies take, but the most common ones are from white to grey or greyish blue, or from shades of pale brown to almost black.

Specific similarities

Both have a MOHS 7 hardness level.
Both are silicate microcrystallines.
Both are glassy, yet waxy, like frosted bathroom glass.
Both fracture conchoidally (breaking in a way that does not follow natural lines of separation).

Agate vs Chert: Differences

On a human level, agate is more highly prized than chert.

It is considered a minor gemstone, and ‘agate’ is often used to name single-coloured stones of gem quality.

Chert is bland and unappealing to the human eye.

Because of impurities, chert is often multicolored.

Agate vs Chert: Why do people confuse them?

Unfortunately, mineralogists put chalcedony into many categories, leading to plenty of confusion.

The result is that often, the same mineral goes by several names.

For example, agate, carnelian, onyx, chrysoprase, carnelian, jasper, sard, flint and chert.

All these chalcedony varieties comprise silica.

However, we call them by different names depending on their geographic location, color and/or color banding, and nicknames.

Another reason people confuse the two stones is that the physical properties of chalcedony occur in narrow ranges.

For example, on Moh’s hardness scale, chalcedony ranges between 6.5 and 7 only.

Chalcedony’s specific gravity ranges between just 2.58 and 2.64.

Unlike hardness and specific gravity, chalcedony varies in size; it can range between small grains and large rocks, which weigh several tons.

This makes it impossible to distinguish chalcedony based on size.

Summary

It is easy to see why people easily confuse the stones.

Agate and chert are both chalcedonies, and the classification of these minerals is a non-standardized horror.

On the whole, though, if it doesn’t look like we could polish a stone into a gemstone we would be happy to wear, it is probably chert.