Agate and jasper are two of the most common types of rock that people go rockhounding for. They can be very attractive with beautiful color, shape and structure, and both are a nice hard material. Sometimes they even form together in the same rock, which are called “jasp-agates.”
These various types of rock can look very different, but they are actually very similar material that naturally form in much the same way. The are all a type of chalcedony.
So, what is chalcedony? The chalcedony is a name given to rock materials that are made up of microcrystalline quartz. The term quartz is used to refer to mineral materials made up of oxygen and silicon (SiO2). The term microcrystalline quartz, therefore, refers to quartz in microscopic crystals form.
Chalcedony rocks are known for their hardness, ranking 7 on the Mohs scale of hardness. For this reasons, chalcedony materials such as agate and jasper are loved by rock tumbling enthusiasts because they will take a nice polish. When forced to break, chalcedony materials usually follows a conchoidal fracture which gives broken pieces a smooth non-granular texture with a vitreous luster.
What is an Agate?
Agate is a form of chalcedony what is usually semitransparent to translucent in color. If you have a piece of a rock that you can see through (even if it’s foggy) then it probably is an agate. The same case applies to a piece of rock that light can pass through even though you may not be able to see all the way through it. However, if a piece of rock completely blocks light from passing through it then it is not an agate.
Agates are often comprised of banded materials which can be identified by closely observing a specimen using a microscope. The pieces that show nice banding are some of the prettiest agates that you can find. However, it is important to note that some agates – especially the translucent ones – may not show bands easy.
Most of the agates found in the world were formed in areas that had volcanic activities. Volcanic activity produces silica. Once the volcanic eruptions were over in a given place and the rain fell on the volcanic ashes the silica in the ashes dissolved in the rainwater. As the silica concentration in the water became high the water turned into a silica gel which was held in cavities in the lava and slowly crystallized into microcrystalline quartz.
If the water-silica solution that crystallized to form the agate had impurities in them then this resulted in translucent agates. Usually, it is these impurities that give the agates their color. Most agates are found formed in igneous rocks that include andante, hyalite, and basalt among others.
What is Jasper?
Jasper is the exact opposite of agate in terms of appearance. Jasper is opaque meaning that you cannot see through it and it does not allow light to pass through it. Generally it has the same structure as an agate but it is completely opaque and more like a typical rock.
Silica, when dissolved in water to form the silica gel, is transparent. However, given that most of the chalcedony rock formation took place in areas where there had been volcanic eruptions and the silica gel was flowing on the ground, the gel mixed impurities from different materials which added color to it. During the crystallization process, the impurities gave color to the microcrystalline quartz formed. It is these impurities in the structure of the jasper that block light from passing through the materials.
Difference between Agate and Jasper
As we now understand, the formation of these two minerals was essentially the same process. It was only the addition of foreign substances during their formation that turned a transparent chalcedony into one that is completely opaque.
You cannot identify an agate or jasper specimen by color alone. Each of these minerals come in a wide array of colors, but it the translucency (or lack of) that determines what type of mineral you actually have. Jaspers are commonly found in red, green, yellow, tan, brown, and even purple.
Agates can be found in pretty much all colors, but certain colors will be more abundant depending on the area that you search. Agates found around the Great Lakes are often a dark reddish color with distinct banding. Along the beaches of Oregon and Washington you will find agates ranging from clear to dark orange. There are even dark agates that are dark blue to nearly black.
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Cutting, Polishing and Tumbling
Both agates and jaspers are very popular with rockhounds because they are hard material that can be used in lapidary. They can be cut and polished resulting in a nice shiny face that will accentuate their internal structure and natural designs.
Pretty much every serious agate hunter has a good rock tumbler. This is probably the most popular thing to do with both agates and jaspers. If you follow the correct procedure they will tumble beautifully and result in nicely polished little stones.
For tumbling, I like to separate out the nice natural pieces and the lower-grade pieces. I like to keep the better specimens natural, while the more boring pieces will go into the tumbler. Often this will improve them drastically, and pieces that weren’t attractive at all will come out of the tumbler completely transformed!
Agate & Jasper Values
The values of both mineral varies considerably on each piece. Since both agate and jasper are relatively common, average pieces don’t generally don’t sell for a lot of money. There are definitely some exceptions though. Certain types of agate and jasper can bring quite a bit of money, particularly if they can be cut and polished.
Larger pieces are always harder to find and as such they will bring a higher value. For example, a small agate the size of a marble might not be worth any real monetary value, but a large baseball size chunk be worth something. Larger material can be cut into slabs and used for lapidary. If a nice piece can be turned into nice cabochons that will certainly increase its value.
Of course, the overall beauty of each piece is the most important variable. More “average” pieces might not be worth much at all, but individual agate specimens with exceptional color, clarity, and nice banding can actually bring quite a bit of money. Again, the larger piece are going to bring the highest premium because they can be used to make things like bookends, spheres, and other collectable items.
There are particular forms of chalcedony that are rarer than others, and as a result they will bring a premium. These generally have a combination of rare colors, distinct banding or other structure, and even visible crystalline formations inside of them. These sorts of variable will add to the value of the pieces that you collect.
Next: Your Complete Guide to Rock Tumbling – Step-by-Step