What Are Agate, Jasper and Chalcedony?
A collection of brightly-colored agates and jaspers of many different kinds. All of them are "chalcedony".
What's the Difference?
If you read about the gem materials used for lapidary work and rock tumbling, you will encounter three names over and over again. These are "agate," "jasper" and "chalcedony." These names are often misunderstood and often used incorrectly.
With a little knowledge you can use these names correctly for most specimens. However, some specimens can be difficult or impossible to name correctly with these terms if you must rely only on visual inspection of the material.
We would like to provide a short lesson on these names to help you understand them and use them correctly - as much as that is possible.
What is Chalcedony?
Chalcedony is a generic name given to materials that are composed of microcrystalline quartz. Agate and jasper are both varieties of chalcedony.
What is microcrystalline quartz? "Quartz" is a mineral composed of silicon and oxygen (SiO2) and the word microcrystalline means that the quartz is in the form of crystals that are so small that a microscope must be used to visualize them individually. Sometimes the word "cryptocrystalline" is used instead of "microcrystalline" but both of these words are imprecise ways of communicating a microscopic size.
Chalcedony is a very hard material. It has a hardness of 7 on the Mohs scale. It breaks with a conchoidal fracture, and freshly broken pieces have a very smooth, non-granular texture and a waxy to vitreous luster. These characteristics enable chalcedony to be cut and polished into a bright, durable gemstone. Varieties of chalcedony are favorite materials for making tumbled stones.
Chalcedony occurs in a wide range of colors. It is commonly gray, white, brown, red, yellow, orange and black, but it can occur in any color. It can also be banded or have plume, dendritic, mottled, mossy or other colorful structures within. At one time the word "chalcedony" was reserved in parts of the gemstone industry for a light blue translucent material; however, this use of the word has nearly disappeared.
A collection of brightly-colored agates! Included in this are banded agates, plume agates, mossy agates, and eye agates in a wide variety of colors. If you are an experienced observer you can tell that many of these specimens are translucent.
What is Agate?
Agate is a translucent to semitransparent chalcedony. If you have a piece that is semitransparent you will be able to hold a very thin piece up and see distorted or foggy images through it. If you hold a translucent piece up to a source of light you will see a small amount of light passing through the thin edges. If you hold it up to the light and pass your hand between the material and the source of light, your hand will block the light passing through the material.
Agate is generally a banded material, and observing bands in a specimen of chalcedony is a very good clue that you have an agate. However, some agates do not have obvious bands. These are often translucent agates with plume-shaped, dendritic or mossy inclusions.
How Does Agate Form?
Many agates form in areas of volcanic activity where waters, rich in dissolved silica (SiO2), flow through fractures and cavities in igneous rocks. When the solution is highly concentrated with dissolved silica, a silica gel can form on the walls of these cavities. That gel will slowly crystallize to form microcrystalline quartz.
Over time, additional layers of gel are deposited and these form younger bands of microcrystalline quartz on the walls of the cavity. If the dissolved mineral composition of the silica-rich water changes over time, impurities (elements other than silicon and oxygen) can be incorporated into the gel and into the microcrystalline quartz. These impurities can alter the the color of the microcrystalline quartz. This can produce the color banding. Crystallization of foreign (non silica) materials is often what forms the plumes, dendrites, or mossy structures that are often seen in translucent agate.
Although agates typically form in igneous rocks such as basalt, rhyolite, and andesite, they can also form in sedimentary rocks such as limestone. All of these types of rock are more susceptible to weathering than agate. So when the rocks are eventually broken down by weathering, the durable agates will remain. This is why agate nodules are often found in stream valleys that cut through fine-grained igneous rocks or limestone.
A collection of jaspers in a wide variety of patterns and colors. If you are an experienced observer you can tell that many of these specimens are obviously opaque.
What is Jasper?
Jasper is an opaque variety of chalcedony. Opaque means that neither light nor images pass through.
Microcrystalline quartz in its pure form is semitransparent. When a small amount of impurities or foreign materials are added, the color of the microcrystalline quartz changes and its ability to transmit light decreases. Jasper contains enough impurities and foreign material to render it opaque. So, the real difference between jasper and agate is the amount of impurities and foreign material contained with a specimen.
How Does Jasper Form?
While agate is typically a material that forms in the cavities of an igneous rock or limestone, jasper often forms when fine particulate materials are cemented by silica. This often occurs in soft sediments when silica precipitates and cements them into a solid mass. These included particles are what give jasper its color and opacity. A sedimentary material known as chert forms in extensive bedded deposits. It is also an opaque variety of chalcedony that can be called a "jasper."
Jaspers are also known to form when volcanic ash or fine pyroclastics are cemented into a solid material from the precipitation of silica from solution. The cementation process is sometimes so aggressive that the sediment, ash or volcanic particles are dissolved or recrystallized into microcrystalline quartz.
The Difference Between Agate and Jasper
The primary difference between agate and jasper is in their diaphaneity. The word "diaphaneity" is used to describe "how easily light passes through a material." There are three general levels of diaphaneity. They are, from highest to lowest:
- Transparent (light and images pass through)
- Translucent (light passes through)
- Opaque (no light passes through)
The System of Assigning a Name
"Chalcedony" is a name that is based upon two things: 1) crystal size, and 2) composition. Chalcedony is a microcrystalline quartz. Easy!
"Agate" is a name based upon three things: 1) crystal size, 2) composition, and 3) diaphaneity. Agate is a microcrystalline quartz with a translucent to semitransparent diaphaneity. Easy!
"Jasper" is a name based upon three things: 1) crystal size, 2) composition, and 3) diaphaneity. Jasper is a microcrystalline quartz with an opaque diaphaneity. It is opaque because it contains enough non-chalcedony material to interfere with the passage of light. Easy.
If you have a piece of chalcedony, determining if it is an agate or a jasper is easy when that material is clearly semitransparent, translucent or opaque. However, it can be difficult to determine the boundary between translucent and opaque. In addition, some specimens can have translucent zones and opaque zones. What are they called? Some people have solved this problem by using the term "jaspagate" or "jasper-agate" when a specimen contains zones of both jasper and agate. Anyone who is correctly using the name "jaspagate" has probably given a rock more than a casual look. :-)
We Aren't Trying to Muck Things Up, But You Might Want to Know....
The names "jasper" and "agate" are often used very generously and sometimes not used when they could be. Here are a few examples:
Dalmatian Stone: A material known as "dalmatian stone" because it is a white rock with lots of black spots, has often been called "dalmatian jasper." However, we sent some out for analysis and learned that it was not jasper at all, but an igneous rock composed of tiny grains of white feldspar and black grains of a hornblende group mineral. Dalmatian stone is often dyed, so if you see a stone with black spots and an outrageous color (like the blue, red, green and purple stones at right) it might be dalmatian stone.
Picasso Stone: This material is known as "picasso stone" because it looks like an abstract painting. Many people also call it "picasso jasper". However, this material is not a "jasper". It is actually a dolomite (a dolomitic marble to be more precise) that is mined in Utah. Dolomite is a carbonate rock that is very different from quartz. It is very soft for a gem material with a hardness of only 4 on the Mohs scale.
Ocean Jasper: The material known as "ocean jasper" is reported to be a silicified rhyolite - another igneous rock. Ocean jasper is a really interesting material mined in the country of Madagascar (an island nation off the southeastern coast of Africa. If you look at it closely, many pieces will contain concentric orbs, translucent banded agate, opaque jasper, and vugs lined with druzy quartz crystals. It is a "jasper" and much more!
Petrified Wood: We don't want to start any arguments, but much of the material called "petrified wood" is composed of chalcedony with an opaque diaphaneity. Shouldn't that make it "jasperized wood" or at least a variety of jasper? Something to ponder. Many people don't realize that petrified wood is found at many locations from around the world. On our tumbling rough page, we sell petrified wood for rock tumbling from Arizona and a mix of petrified wood from many locations in the United States.
Green Tree Agate and Green Moss Agate: These materials are often genetically related - that means they form under similar conditions. Green moss agate is a semitransparent agate with green mossy inclusions inside. Green tree agate is a white jasper with green mossy inclusions inside - however they are only visible where they are exposed at the surface. Green moss agate and green tree agate have been found together in the same deposit - having formed just a short distance from one another.
Bumblebee: This exceptionally colored material is often called "bumblebee jasper" or "eclipse jasper". It forms on the island of Bali near the hot vents of an active volcano named Mount Papandayan. As a banded material, and for that reason, some people want to call it "agate." It is an opaque material, and for that reason, some people want to call it jasper. However, it is neither. It is a lithified sediment that contains a volcanic brew of materials that include: volcanic ash, gypsum, barite, sulfur, and even some orpiment (an arsenic mineral!). It is often cut into cabochons, but much of what is cut is stabilized with a resin because it is soft, porous and fragile. Not recommended for rock tumbling!