Agate, Jasper, Tigers Eye and other Varieties of Chalcedony

Many of the finest and attractive gems come from the chalcedony family that includes a wide variety of cryptocrystalline quartz, or dense silica minerals whose structure cannot be resolved without an optical microscope. These varieties of silica are particularly popular in lapidary and well known by rockhounds worldwide.

The literature provides confusing categories for chalcedony. The problem arises due to a variety of labels applied to the same mineral, such as agate, carnelian, chrysoprase, onyx, sard, jasper, chert, flint, youngite, etc. These varieties of chalcedony, consist of silica (SiO₂) just like quartz; but they are identified based on color, geographical location, color-banding and namesakes (Hausel, 2014).

It would make more sense to call cryptocrystalline silica “chalcedony” and then add color modifiers: i.e., ‘red chalcedony’ for ‘jasper’, ‘green chalcedony’ for ‘chrysoprase’, etc. The current system has many names to different colors. But, since these are ingrained in the literature, we will stick to the familiar terminology.

Chalcedony consists of microscopic fibrous quartz with minute pore spaces filled with water, air, or mineral particles that produce attractive colors and bands in the variety known as agate. Chalcedony consists of granular quartz with roughly equal-dimensional micro-crystals rather than fibrous micro-crystals such as jasper, flint, and chert. The distinction between fibrous and granular cryptocrystalline quartz is not universally recognized as differences are microscopic.

Jasper breccia from Tin Cup district, Wyoming. Photo by the
author.

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 and 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, typically, slightly lower than quartz (H=7) depending on porosity and purity as related to the fibrous structure. In other words, chalcedony will scratch your car’s windshield and other types of glass.

The specific gravity of chalcedony is 2.58 to 2.64, or slightly lower than coarsely crystalline quartz because of porosity in chalcedony. Being light, it will easily wash out of a gold pan. Chalcedony can vary in size from grains to masses weighing tons.

Jasper is a variety of deep red, reddish-brown to yellow-orange chalcedony. Jasper is essentially indistinguishable from sard and carnelian in that jasper is opaque while sard (reddish to reddish-brown) and carnelian (reddish-orange to orange) are considered to be transparent to translucent chalcedony.

Sweetwater dendritic agate, Granite Mountains, Wyoming. Photo by the author.

India is the principal source of carnelian, but other sources include Arizona, Colorado, Michigan, Washington and Wyoming. Sard is chalcedony that is primarily colored by goethite (hydrated iron oxide) and is gradational with carnelian and jasper. Sard is translucent to nearly opaque and brown, brownish-red, and brownish-yellow. These colors are a result of trace iron-oxide (rust) in the minerals. 

Jasper is sometimes found in large quantities and sometimes found in hydrothermal gold deposits. Tons of jasper material were found adjacent to the Dry Creek Road leading into the Rattlesnake Hills gold district in Wyoming, where the lapidary gem caps two low-lying hills known as Jasper Knob and South Jasper Knob. These could be used in jewelry, statuary, decorative stone and even countertops.

The knobs include considerable red, reddish-brown, tawny, to yellow-orange chalcedony (jasper, sard and carnelian). Never tested for gold, but because of close proximity to the Rattlesnake Hills gold district, they should be, particularly because fossil leaf imprints suggest the jasper was deposited in surficial, silica-rich, mud in a hydrothermal spring. Jasperoids discovered in the Drum Mountains of Utah yielded significant gold anomalies, as reported by the US Geological Survey years ago. Thus some jaspers and jasperoids (jasper-like material) are worthy of gold assays.

Another jasper deposit in the Tin Cup district northwest of Jeffrey City in central Wyoming contains considerable tonnage. Searcj Google Earth, search for ‘Jeffrey City, WY’ - the district lies north-northwest of Jeffrey City (42^o38’55.40”N; 107^o53’06”W) in the middle of jade country. Tin Cup was prospected in the 19^th century and promoted as a gold district. However, my investigations of the old prospects found no detectable gold; thus much of the past gold promotion was likely a mining scam. Even so, the district produces considerable jasper and jasper breccia.

The very popular Dryhead agate from Montana. Photo by the author

Some 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 of the popular Sweetwater moss agates from the Granite Mountains in central Wyoming fluoresce brilliant yellow due to presence of hydrous uranium arsenate. Some of the opal and agate in the Cedar Rim opal field to the east, fluoresce lavender to white.

Chatoyancy (fibrous optical reflectance caused by silky fibrous structure) is displayed by some agates and can be attractive in polished stones. Adularescence is rare but is found in some chalcedony and opal. Violet adularescent chalcedony is described in some specimens from Iran. Adularescence is an optical feature commonly referred to as ‘schiller’, and 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 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 straight parallel layers in onyx. Typically, agates produce a variety of color bands and may grade into other forms of chalcedony. 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:

• 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.

Fairburn (fortification) agate, South Dakota. Photo by the author

• 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. Other forms contain patches to masses of moss green chalcedony.
• Botryoidal agate – agates that exhibit botryoidal texture that exhibits, surficial, hummocky to rounded forms similar to bunches of grapes.
• Flame agate - agates with red to orange flame-shaped dendrites.
• Iris agate – agates with spectral display of colors due to microscopic diffraction grating caused by alternating bands of material that has higher and lower refractive indices.

Goniobasis agates from Delany Rim, Wyoming.
Photo by the author

Some agates are provided 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 is found recovered include Riddle, Oregon, Tulare County, California and Wyoming. Chrysoprase, also referred to as prase agate includes beautiful, light-green agates commonly referred to as prasiolite. Such agates are found in Poland, Brazil, Thunder Bay Canada, and the Granite Mountains of Wyoming. This can produce a very attractive gem when polished to highlight its green color and translucence. But in places like the Granite Mountains, one must verify that the stone is either chrysoprase, serpentinite, or jade, as all three have similar appearances but differ by hardness, magnetism, and specific gravity.

Onyx is made up of alternating dark and light-colored, parallel bands or layers of chalcedony. The 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 a group of deposits discovered on the top of Quaking Asp Mountain south of Rock Springs, and for another deposit in the Tin Cup district.

Eocene-age, Petrified Sequoia tree in Wasatch Formation, near
Buffalo, Wyoming. Photo by Wayne Sutherland.

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 leaving hard, resistant pseudomorph that can contain extraordinary details of the original plant or 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.

Another form of chalcedony, known as tiger’s eye, is an agate with distinct chatoyancy, and can occur as golden yellow on a brown background. 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.

Blue forest fossil wood, Big Sandy Opening, Wyoming. 
Photo by Wayne Sutherland

The chatoyancy in tiger’s eye is often cited as being a result of pseudomorphic replacement of asbestos-form minerals, such as crocidolite. Tiger’s eye is developed by vein-filling process in which the asbestos fibers were replaced by overgrowths of chalcedony. Quartz provides the hardness for the mineral, and the crocidolite is responsible for the chatoyance. Most tiger’s eye is recovered from South Africa, although some has been found in lesser deposits in California (USA), Australia, India, Myanmar, and Namibia.

HOW TO IDENTIFY CHALCEDONY

Chalcedony has a Moh's hardness very close to crystal quartz (H=6.5 to 7), so it will scratch window glass. It also produces distinct conchoidal fracture - just like glass. It also has glassy to waxy luster, low heft (SG=2.59 to 2.61) and will typically be attached to a rock hound. Being cryptocrystalline, it will not have any district crystal form, except where it has pseudomorphed another mineral. Being light weight, it will easily wash out of a gold pan. Chalcedony can vary in size from grains to masses weighing tons.