Strictly speaking, most chalcedony (kal SED' uh nee) is a mixture of microcrystalline or cryptocrystalline quartz and moganite. (Some specimens of this material are free of moganite). In common practice, only the translucent, single-color types of this material are sold under the name “chalcedony.” The rest of this group are sold under individual variety names or as agate or jasper. Although the definitions of these types overlap, agate is defined either by its translucency, color patterns, or inclusions. Jasper usually refers to an opaque, solid colored stone stained by oxides.
General high value points for all chalcedonies would be strong color saturation and high translucence. The value of varieties such as agate and carnelian is more a function of the beauty of their cutting than the material itself.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Quartz|
|Varieties||Agate, Fire Agate, Iris Agate, Binghamite, Bloodstone, Carnelian, Chrome-Chalcedony, Chrysocolla Chalcedony, Chrysoprase, Jasper, Dallasite, Orbicular Jasper, Plasma, Onyx, Sardonyx, Petrified Wood, Pietersite, Sard|
|Refractive Index||1.540 - 1.553|
|Luster||Waxy to dull|
|Polish Luster||Greasy to vitreous|
|Fracture Luster||Waxy to dull|
|Specific Gravity||2.55 to 2.91|
|Cleavage||None. Cryptocrystalline quartz: tough.|
|Luminescence||Varies widely in cryptocrystalline quartz due to traces of impurities.|
|Enhancements||Dyeing: common, stable. Detected by appearance, spectrum, or Chelsea filter. Heating changes color, common, stable, undetectable.|
|Special Care Instructions||None|
|Transparency||Transparent to Opaque|
|UV Long||Usually inert|
|UV Short||Usually inert|
|Absorption Spectrum||Dyed green gems show weak lines at 6450 and 6700 nm. Dyed blue gems at 6270, 6660, and 6900. Others not diagnostic.|
|Identifying Characteristics||Dyed blue and green gems show pink in Chelsea filter.|
|Optics||o = 1.540; e = 1.553. Uniaxial (+)|
|Etymology||May refer to the ancient seaport of Chalcedon, now located in Istanbul, Turkey.|
|Occurrence||Deposits in a wide variety of environments. Nodules, veins, crusts.|
The colors of chalcedonies are a result of metallic impurities (such as iron, nickel, copper, and titanium) present during crystallization. Unstained material is often grayish blue. Chalcedony is a tough gemstone, good for all jewelry applications. Pieces are usually cabbed or carved, although an exceptional, near transparent piece may be faceted.
Varieties of Chalcedony
This material is the darling of some gem carvers and jewelry designers because of its varied and ethereal blue colors.
The color varieties are generally designated by place names. They vary in depth of blue color and the degree to which the blue is modified by gray or pink hues. As an overall group, blue chalcedony varies from pale to medium tones and in degree of translucency. Some pieces have a slight adularescence that enhances their value. This phenomenon is due to light interference from layers of microscopic inclusions. The effect is like a shimmering, floating, interior light.
“Mohave” and “Mt. Airy” blues originate in California and Nevada, respectively, and are slightly to moderately grayish blue with a light to medium color range.
Blue chalcedony from Namibia, often called “African Blue,” varies from grayish to nearly pure blue and from light to medium dark.
The most unusual type, and arguably the most valuable, is from Oregon. Its blues are modified by slight to moderate amounts of pink, making a noticeably lavender gem. Nonetheless, it’s called “Holly Blue.”
Translucent to semi-opaque, red, orange-red, or brownish chalcedony.
Similar to carnelian, but with a brownish tone. Its transparency ranges from slightly translucent to opaque.
Green, or yellowish green chalcedony.
A dark green variety, frequently found with white or yellowish spots. Plasma is opaque due to densely packed actinolite crystals. Bloodstone, or heliotrope, is plasma with red and orange spots of iron oxide.
A chalcedony with straight bands of colors. Black onyx has black and white layers. It occurs in nature in thin bands. What you find in the stores is almost always dyed. Sardonyx is onyx with white and red layers.
Flint and Chert
Opaque, dull gray or white chalcedony. They rarely make an appearance as gems, but have been used for arrowheads, driveways, and other utilitarian purposes. These materials are very compact and hard. Mozarkite is a colorful type of chert that has been fashioned into jewelry.
Silicified dinosaur bone is primarily chalcedony in composition. Gem cutters can cut and polish them like any other quartz family gem. This material can show interesting patterns and colors such as brown, red, pink, blue, purple, green, orange, and others.
Translucent green chalcedony colored by nickel. Chrysoprase material may resemble fine jade.
Chatoyancy is a type of sheen that creates an “eye-like” optical phenomenon. Chalcedonies that show this effect are called hawk’s eye if the stone is blue, tiger’s eye if the stone is brown or gold, and zebra tiger’s eye if the stone is blue and gold.
A purple-colored variety from Arizona has been marketed under this trade name. The material occurs in veins and blocks up to 1 m thick. Masses over 100 kg have been recovered. Its properties are normal for chalcedony (RI = 1.54, SG = 2.61), and its coloration appears to be the same as amethyst.
Chrysocolla in Quartz
A tough, siliceous material consisting of blue chrysocolla in fine particles disseminated in silica. This is a rich blue, hard material that takes an excellent polish.
For information on synthetic quartz in general, consult the quartz gem listing.
Almost any chalcedony can be turned red by heating in an oven since it contains finely disseminated iron compounds that are oxidized by heating.
More detailed source information is included in the individual gem listings for the chalcedony varieties.
- Chalcedony: California; Iceland; India; Mexico; Namibia; Nevada; Oregon; Russia.
- Carnelian: Brazil; Egypt; India; Uruguay.
- Sard: Brazil; Uruguay.
- Bloodstone: Australia; Brazil; India; United States.
- Chrysoprase: Western Australia (Yandramindra, Wingelina, Kalgoorlie); Brazil; California; Russia.
- Chrysocolla in Quartz: Arizona; New Mexico; Mexico.
- Dinosaur Bone: Colorado; Utah; Wyoming.
Chalcedony is usually nodular, but masses can be several pounds and many inches in diameter.
Chalcedony requires no special care. Mild detergent, warm water, and a soft brush are a good choice for cleaning.
by Dr. Joel Arem ; Donald Clark, CSM IMG; Barbara Smigel, PhD GG