Agate

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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 any material that is 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 smaller than 30 microns in size (a micron is a unit of measure that is 1/1000th of a millimeter). These are very tiny quartz crystals – smaller than can be seen by the unaided eye. (Sometimes the word “cryptocrystalline” is used instead of “microcrystalline.”)

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.

Chalcedony occurs in a wide range of colors. It is often 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 color patterns. 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.

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)

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, or instead of being banded they have plume or mossy inclusions.

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.

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, plumes or moss 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.

What is Jasper?

Jasper is an opaque variety of chalcedony. 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 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 particulates are what give jasper its color and opacity. A sedimentary material known as chert forms in extensive bedded deposits, and as an opaque variety of chalcedony it receives the name “jasper.” Jaspers are also known to form when volcanic ash is cemented into a solid material from the precipitation of silica from solution.

Problems With Assigning a Name

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” when a specimen contains both jasper and agate.

Assigning a name can be difficult when the composition of the material is unknown. A material known as “Dalmatian stone” 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. The material known as “ocean jasper” is reported to be a rhyolite – another igneous rock.

Happy Tumbling

Source
http://rocktumbler.com/blog/what-is-agate-jasper-chalcedony/