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
See the gem listings for agate and jasper for detailed information on those types of chalcedonies.
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.
Dyeing is an ancient and common practice for enhancing chalcedonies. These stones are relatively porous. This is usually a stable process. Dyeing enhancements should be disclosed to consumers.
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.