Chalcedony (Agate, Jasper, Chert and More)
Cryptocrystalline quartz or chalcedony includes dense varieties of silica whose structure cannot be resolved without an optical microscope. These varieties of silica are particularly popular for lapidary. You will want to also review my page on quartz.
Mineralogists and rock hounds provide confusing categories for chalcedony. Much of the problem is due to numerous names applied to the same mineral such as agate, carnelian, chrysoprase, onyx, sard, jasper, chert, flint, youngite, etc. These are all varieties of chalcedony, and all are formed of silica (SiO2) just like quartz; but they have been given different names based on color, geographical location, color banding and namesakes (Hausel, 2014).
It would be a easier to simply call all cryptocrystalline varieties of silica “chalcedony” and add a color modifier – such as ‘red chalcedony’ for ‘jasper’, ‘green chalcedony’ for ‘chrysoprase’. It’s like taking sapphire and applying dozens of names to different colors of sapphire when all one needs to do is simply call green sapphire, “green sapphire” and pink sapphire “pink sapphire”.
Chalcedony consists of microscopic fibrous quartz with minute pore spaces filled with water, air or colored mineral particles that produce attractive colors and bands in the variety known as agate, or it consists of granular quartz with roughly equidimensional microcrystals rather than fibrous microcrystals, such as jasper, flint, and chert. The distinction between fibrous and granular cryptocrystalline quartz is not universally recognized and such differences are microscopic.
Flint is dark-brown to black chalcedony - the color caused by impurities. Chert is commonly opaque, light-gray to white chalcedony. So we can think of these two as the Yin/Yang of chalcedony – one black and the other white with all shades in-between.
Chalcedony often occurs as cavity fillings, linings, replacements and fracture fillings. It may be found with quartz crystals and/or drusy quartz in geodes. It is also found as fracture fillings and replacements of organic material such as petrified wood. It has no cleavage and will break with uneven rough to splintery or conchoidal fracture similar to glass (Hausel, 1986, 2009).
The hardness of chalcedony is 6.5 to 7 on the Moh’s hardness scale. The hardness may appear to be lower than quartz (7) depending on porosity and purity as related to the fibrous structure. In other words, the mineral will be able to scratch your car’s windshield. Some still think only diamond will scratch a windshield, but if you have ever been in a dust storm around Phoenix, you might take a close look at your windshield because the quartz dust (sand) likely pitted your softer windshield glass.
The specific gravity of chalcedony is 2.58 to 2.64. This is slightly lower than coarsely crystalline quartz because of slight porosity in chalcedony. Being so light, it will easily wash out of a gold pan. Chalcedony can vary in size from grains to large masses weighing tons.
Jasper is a variety of deep red, reddish-brown to yellow-orange chalcedony. Jasper is essentially indistinguishable from sard and carnelian other than jasper is opaque while sard (reddish to reddish-brown) and carnelian (reddish-orange to orange) are considered to be transparent to translucent chalcedony.
India is the principal source for carnelian, but other sources include Colorado, Michigan and Washington. Sard is chalcedony that is primarily colored by goethite (hydrated iron oxide) and is gradational with carnelian and found in some jasper. Sard is translucent to nearly opaque and occurs in brown, brownish-red, and brownish-yellow colors.
The color in these varieties is due to presence of trace iron oxide (rust), whether it occurs as hematite, limonite or goethite. Jasper is sometimes found in large quantities. At one deposit I examined years ago, tons of this material were found adjacent to the Dry Creek Road leading into the Rattlesnake Hills gold district in Wyoming. The jasper caps two low-lying hills known as Jasper Knob and South Jasper Knob. These could be used to produce considerable jewelry, statuary, decorative stone and even countertops, but they just sit there collecting dust.
The knobs contain considerable red, reddish-brown, tawny, to yellow-orange chalcedony (jasper) and some specimens have excellent leaf imprints. These rocks were never tested for gold, but because of their close proximity to the Rattlesnake Hills gold district west of Casper, they should be tested, particularly because the fossil leaf imprints suggest the material was deposited at the surface in silica-rich mud from a hydrothermal spring. To see this area, search for “Dry Creek Rd, Sweetwater, Wyoming” on Google Earth and this will take you 15-miles northwest of the Jasper Knobs. Similar jasperoids in the Drum Mountains of Utah were discovered to yield significant gold anomalies by the US Geological Survey several years ago. Thus many jaspers and jasperoids (jasper-like material) are worthy of gold assays. Since the Jasper Knobs were described, the Rattlesnake Hills gold district has picked up considerable interest for gold now that a major gold deposit has been intersected by drilling. The area is heavily staked.
Another jasper deposit was found in the Tin Cup district northwest of Jeffrey City in central Wyoming. To visit this area on Google Earth, search for ‘Jeffrey City, WY’ and the district is 11 miles north-northwest of Jeffrey City (42o38’55.40”N; 107o53’06”W) in the middle of jade country. The Tin Cup district was prospected in the 19th century and promoted as a gold district. However, my investigations of the old prospects identified no detectable gold, thus much of the past gold promotion might have been related to a mining scam. Even so, the district has tons of jasper and jasper breccia with potential to produce large tonnages of jasper and some onyx.
Pure chalcedony may fluoresce blue to white. Fluorescence in other varieties of chalcedony may range from null to strong yellow to blue-white depending on the presence of chemical impurities or mineral inclusions. Some popular Sweetwater moss agates from the Granite Mountains of central Wyoming fluoresce brilliant yellow due to presence of hydrous uranium arsenate, and opal and agate from the Cedar Rim opal field tend to fluoresce lavender to white.
Chatoyancy (fibrous optical reflectance due to silky fibrous structure) is displayed by some varieties of agate and can be beautiful in polished stones. Adularescence is rare but is found in some agates and opal. Violet adularescent chalcedony was described in some specimens found in Iran. Adalarescence is an optical feature, commonly referred to as schiller, which produces a bluish luster caused by the interaction of light with internal mineral structures and inclusions. It is a milky scheen, or wavy glowing light effect that appears to originate just beneath the surface of a polished stone.
Agate is defined as banded chalcedony that is found primarily in nodules. But the term agate, has is also been used for other varieties including chalcedony where banding is not evident such as moss agate. It is different from onyx in that agate has curved or irregular banding in contrast to the more straight parallel layers in onyx. Typically agates produce a variety of color bands and
Most agates originate as cavity linings and fillings in a variety of host rocks. Common usage also applies the term agate to varieties of chalcedony that show no banding. Agates are numerous with many names. Here are a few:
- Banded agate – agate with distinct color banding (this is the primary definition of agate).
- Fortification agate – banded agate that flows outward into several points within a nodule to provide an appearance similar to a medieval fortress.
- Eye agate – agate with concentric banding surrounding a point in the center that gives the appearance of an eye.
- Agate breccia – an agate formed of broken lithic fragments that are rehealed by chalcedony and or quartz such as the popular Youngite agate found north of Wheatland, Wyoming.
Moss agate - a translucent chalcedony that encloses moss-like manganese or iron oxide dendrites such as the Sweetwater agates.
- Botryoidal agate – agate that exhibits botryoidal texture that appears as an external hummocky to rounded form similar to bunches of grapes.
- flame agate - dendritic agate with red to orange flame-shaped dendrites
- Iris agate – agate with spectral display of colors due to microscopic diffraction grating caused by alternating bands of material that has higher and lower refractive indices.
Some agates are given local names, such as Youngite (pink to cream limestone breccia clasts rehealed with bluish-gray chalcedony and drusy quartz) from Hartville area, eastern Wyoming, or the Sweetwater moss agates. Others such as the Fairburn agate in South Dakota are popular banded agates. Bloodstone agate is a green opaque chalcedony with red spots that is also known by its earlier Greek name, heliotrope.
Apple-green to light-turquoise green chalcedony colored by garnierite (nickel-silicate) is known as chrysoprase agate. Chrysoprase forms in veins in nickel-rich host rocks such as serpentinite. Some localities where chrysoprase has been recovered include Riddle, Oregon, Tulare County, California and Wyoming. Prase agate (referred to as prasiolite if specimen is deemed quartz) is a green agate found in Poland, Brazil, Thunder Bay Canada, and even in the Granite Mountains of Wyoming. This can produce a very attractive sone when polished to highlight its green color and translucence.
Onyx is made up of alternating dark and light colored straight parallel bands or layers of chalcedony. This hard chalcedony onyx is similar in appearance to soft marble onyx or Mexican onyx. Mexican onyx is considerably softer and is easily scratched. Attractive specimens of onyx marble were described in Wyoming in the Hartville uplift. The first known reports of chalcedony onyx in Wyoming was by the author for a deposit discovered on the top of Quaking Asp Mountain south of Rock Springs, and for another deposit found in the Tin Cup district.
Petrified (fossilized) wood is produced by silica-rich groundwater replacement of buried organic trees and limbs. Supersaturated silica solutions tend to slowly replace organic material of entire plants and trees leaving a hard and resistant pseudomorph that can contain extraordinary details of the original tree all the way down to cellular structure. Cryptocrystalline quartz of many types, including agate and jasper, may be found as petrified wood.
Petrified wood is found on all of the continents with spectacular examples in the Petrified Forest National Monument and surrounding areas in northern Arizona where Triassic Shinarump and Chinle Formations contain numerous petrified wood tree trucks scattered all over the surface. Petrified wood is also known in the Eden Valley and Blue Forest areas of southwestern Wyoming, from the Wiggins Fork area in Absaroka Mountains of Wyoming, and from Yellowstone National Park in northwestern Wyoming.
Tiger’s Eye is an agate with distinct chatoyancy, and can occur as golden yellow on a brown background. But depending on the background or base color, these agates receive various gemological and rock hound terms. When the background is greenish-gray or green the rock may be known as cat’s eye. When blue-gray to blue, it is known as hawk’s eye, and a stone with mahogany color base is called bull’s eye. The chatoyancy is usually enhanced in rounded, polished, ornamental stones or cabochons.
The chatoyancy in tiger’s eye is often cited as being caused by pseudomorphic replacement of asbestos-form minerals such as crocidolite. Tiger’s eye is developed by vein-filling process in which crocidolite asbestos fibers are replaced by overgrowths of chalcedony. The quartz provides a relative hardness, and the crocidolite is responsible for the chatoyance. Most tiger’s eye comes from South Africa although it has also been found in lesser deposits in California (USA), Australia, India, Myanmar, and Namibia.