Jaspers!
photographs by Steve Blyskal
Many rockhounds regard jasper as a sort of inferior agate, at least until they pay premium prices for a beautiful specimen or slab of rough. Much of the information I will cover in the article comes from a great book in the HGMS library, Agates and Jaspers, by Ron Gibbs, but you can’t check it out until I return it! It contains many definitions and even prices as of 2009. Other great information can be found at various internet sites.
Jasper has been known by many different names since ancient times and was utilized by the Egyptians, Sumerians, Greeks, Romans and others. Two stones in the high priest Aaron’s breastplate mentioned in the Bible may have been jasper. It is popular for metaphysical purposes and there is much information of that source available. It is easy to find since people are selling it! Be aware that many New Age practitioners call any rock they use a crystal, which jasper is not.
“Chert” has often been used as a general term for both flint and jasper. Flint forms in nodules or beds in sedimentary rocks and so do many jaspers. A better characterization for rockhounds would be to call both agates and jaspers variants of chalcedony, which is defined as cryptocrystalline quartz, silicon dioxide or SiO2. For lapidary uses, both take a good polish, they are often colorful with interesting patterns, and they are hard enough to cut and polish or tumble, around Mohs Hardness 7. By definition jasper is also opaque. Both break with a conchoidal fracture with a dull or waxy luster since fractures cross over thousands of microscopic quartz crystals. Quartz crystals, when fractured, have a glassy or vitreous luster.
In Agates and Jaspers Ron Gibbs defines jasper as particulate matter cemented by silica. The sediments can be from common sedimentary cycle weathering, chemical precipitation or igneous ash tuffs or flows. The silica comes from the weathering of rock containing silica and some organic sources, and possibly directly from ash falls. Soft sediments like ocean bottom silt can be cemented by silica from volcanic ash or the breakdown of diatoms or sponge spicules. You could say that all jaspers result from sedimentary processes as long as you are willing to call volcanic ash flows sedimentary! The patterns in some jaspers are due to turbulence in ash flows as they are deposited. Weathering, metamorphism, metasomatism (the introduction or removal of chemicals by fluids) or brecciation due to tectonic activity with re-cementation. They can literally be any color, although red and brown are common.
The subsequent crystallization of the silica cement does not form the fine needles seen in agate due to the high percentage of the particulate matter. Different particulates can give the jaspers in the process of forming, which need to be subjected to heat and pressure to form hard rock, their color, but so can contaminates in silica-saturated hydrothermal fluids or post-formation recrystallization or staining. This can happen with wood, bone, coral, and other organic substances, which can be regarded as jaspers; petrified wood can be very fine grained and lose all appearance of wood grain or its detail may be preserved.
Basically, Chalcedony consists of microcrystalline quartz.
Agate consists of microcrystalline quartz that is semi-transparent or translucent.
Jasper consists of microcrystalline quartz that is opaque.
Jaspers and agates can also grade into one another in the same rock or exist very close to each other; a jasper can have streaks of translucent agate, and an agate can have opaque streaks of jasper. Many lapidaries get around this fact by calling such rocks “jaspagate.”
Agates form in vesicles or cracks in other rocks, such as basalt gas bubbles, faults in rhyolite or sedimentary limestone vugs or replacing organic material in sediments. Jaspers have been found in nodules and cracks, thunder eggs and as massive rock. At the Kokernot o6 Ranch near Alpine, Texas, I have observed agate and jasper nodules in the same basalt, still attached to the local basalt formation. In this case, vesicles in the basalt were possibly filled at different times and subjected to different fluids containing silica from ash and weathering of rhyolite. They may have also been altered at a later time.
What are some popular jaspers? Fine-grained porcelain jaspers are formed from fine-grained claystone or volcanic ash cemented by silica and subjected to metamorphism, metasomatism, tectonic activity and weathering.
The “fine jaspers” are all porcelain jaspers, extremely fine grained and able to take a fine polish:
Imperial Jasper (Zacatecas, Mexico)
Bruneau Jasper (Bruneau, Idaho)
Morrisonite (Malheur County, Oregon)
Willow Creek Jasper (Eagle, Idaho)
Blue Mountain Jasper (Blue Mountains, Oregon)
There are literally hundreds of jaspers and many variations known, due to slight differences in deposition of particulates and cementing fluids and subsequent metamorphism, for example Imperial Jasper, Green Imperial Jasper, Spiderweb Imperial Jasper, Royal Imperial Jasper, etc., all found in nearby localities in a small area. Some jaspers are named for their appearance, including color and geometric and natural-appearing features, some for their localities and some out of purest imagination.
Some common types of jasper aren’t really jaspers at all.
- Dalmation jasper, common in West Texas, is an igneous rock composed of white or tan feldspar with black dots of hornblende.
- Ocean Jasper, found in Madagascar, may be a silicified rhyolite and in some cases is more of an agate.
- Bumblebee Jasper or Agate is a fumarole material from an Indonesian volcano, Mount Papandayan. The yellow and orange are from arsenic minerals.
- Leopardskin Jasper is a spherulitic rhyolite, as are other orbicular rhyolites called jaspers.
Do we have local jaspers? Yes, there is a red jasper found in the gravels of the Brazos, Colorado and Trinity Rivers. I hope to find out more about this jasper and its composition and source.
I believe jaspers are a fascinating subject on which little research has been done and potentially a very rich source of material to talk about. And cut! I expect to write several future articles on jaspers.
References:
https://www.gemstones.com/gemopedia/jasper
https://rocktumbler.com/blog/what-is-agate-jasper-chalcedony/
https://en.wikipedia.org/wiki/Jasper
All photographs by Steve Blyskal