CRYSTALLOGRAPHY Hexagonal, microcrystaline.
REFRACTIVE INDEX 1.544 – 1.553
SPECIFIC GRAVITY 2.651
HEAT SENSITIVE No
SPECIAL CARE INSTRUCTIONS None
ENHANCEMENTS May be dyed.
*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It! For more details see the article on “Hardness and Wearability.”
Technically, chalcedony (kal SED’ uh nee) is any form of microcrystalline or cryptocrystalline quartz, (meaning any form of quartz whose crystals are too small to be seen without high magnification.)
In common practice, only the translucent, single color types are sold as “chalcedony” whereas the rest of this group are sold under individual variety names, or as jasper or agate. While the definitions overlap, jasper usually refers to an opaque, solid colored stone. Agate is defined either by its translucency, or by having a pattern to its colors.
Agate is distinguished by having multiple colors. While not usually as rich as our crystalline gems, the colors can be quite vivid. Agates are sometimes opaque, but they are frequently translucent, and occasionally completely transparent.
Banded agates are some of the most popular. They are found all around the world, with Brazil being one of the most productive sources. Note that many of the richly colored, banded agates you see for sale are dyed.
Lace agate is noted for its delicate designs. Mexico is one of the premier sources for this material.
Dendritic, Moss and Plume Agates
These agates have in common that they contain mineral inclusions which may be any color, but share a roughly tree-like or branching form (dendron = tree). Those which have a more plant or feather-like appearance have been called moss or plume agates, respectively. All agates take a wonderful polish and are tough enough for most jewelry uses. Designers often take advantage of the intriguing patterns these stones have to offer.
Some of the most treasured gems are those that show a picture that appears to be taken from nature. Oregon’s Biggs Jasper is now the most common source. Bruneau Jasper, from Bruneau Canyon, Idaho, used to be the preferred material. Gems from this locality frequently had blue “skies” which the Oregon material lacks. Unfortunately, a dam has submerged the mining site and the material is now quite rare.
In general, agate prices are quite modest with most of the price paying for the fashioning rather than the material itself. In the case of these agates, those with particularly distinctive or landscape like patterns, or those of especially large size, are at a premium. Sinkankas lists wholesale values of commercial grade standard cabs as ranging from $.50 – $20 each, depending on size. Custom cutting or pieces from collectible locations would be substantially more expensive. Especially fine patterns are cited as bringing up to $200 per piece.
My appreciation for fire agate has taken time to reach its current high level. Most of the pieces I saw early on were poorly fashioned and of low quality, and frankly, I was not impressed. Since starting this web site, however, I have had the opportunity to see some outstanding specimens and, as a result, my enthusiasm has increased dramatically. Fire agate is a brown, microcrystalline quartz which has a botryoidal, (grape-like,) growth form. It contains layers of plate-like crystals of iron oxide, (limonite,) in various planes within it. The iridescent colors of red, gold, green and rarely, blue-violet, result from interference between light rays traveling through these thin layers. (We see the same effect when looking at the rainbow colors at the surface of an oily puddle of water; or in the “orient” created by the layers of nacre on the surface of pearl.)
Usually, fire agate pockets occur within specimens of colorless, white, or light gray chalcedony. Fire agate is found only in the American Southwest and Mexico and was not brought into commerce until after World War II. This, combined with the fact that it is one of the most difficult cab materials to cut properly, keeps it scarce and mostly unknown to the general public.
In order to best reveal the colors, the overlying layers of chalcedony must be removed from the botryoidal surface creating a freeform shape with a carved upper surface. Such treatment requires substantially more time per piece and tends to elevate cost. This type of fashioning also leads to a lack of calibrated pieces and has prevented the use of this gem in mass produced jewelry items. Good fire agates are as impressive in their color-play as fine black opal, but far less expensive. Additionally, fire agate is as hard and durable as any quartz making it wonderful for jewelry uses, including rings. The colors and form are rich and dramatic and generally appeal strongly to men (although I can personally attest to their appeal to women!)
The most desirable pieces show color over the entire surface with no dead spots. Red color is the most highly valued, but the few pieces with a sort of lavender-blue are also sought after. Federman lists the wholesale value of the most desirable pieces at a maximum of $20-$25 per carat.
Jasper is an opaque, solid or patterned variety of cryptocrystalline quartz which consists of very tiny quartz crystals colored by various mineral impurities. The names of various jaspers can come from their color: bloodstone, green, lemon; from their pattern: orbicular, poppy, leopardskin, landscape, Picasso; or from a place name: Morrisonite, Mookite.
All types take an excellent polish, are trouble free to care for, and hardy enough for all jewelry uses. These stones are usually cabbed, sometimes carved, and seldom faceted.
Jewelry use of jaspers goes back into the early history of civilization. Various forms of this material are also frequently made into decorative objects, such as ashtrays or bookends. Jaspers are found all over the world, with certain colors or patterns unique to particular locales. Most bloodstone comes from India, all Mookaite from Australia.
Jaspers, in general, are very common; hence most of the value in a given piece relates to the saturation of its color, the beauty of its pattern or the artistry with which it is fashioned. Some types such as Imperial Jasper and Madagascar Jasper do command premium prices as they are relatively rare. In rock shops, pieces of commercial quality cut in simple shapes might be had for $5 or less. Fine material, cut in designer forms, generally ranges between $2 and $5 per carat.
Crocidolite, (blue asbestos,) alters to quartz, but while retaining its fibrous structure. This material is frequently stained by iron, giving it a golden brown color. We know this material as tigers eye. Unstained pieces, retaining their original blue color, are called Hawks Eye. There are also pieces with both colors.
In this description, chalcedony will mean any translucent, cryptocrystalline quartz with a single color, whether it has a special variety name or not. The various types differ in color due to metallic impurities, such as iron, nickel, copper, and titanium present during crystallization. This group of stones is usually cabbed or carved, although an exceptional, near transparent piece may be faceted. Chalcedonies are tough gems, good for all jewelry applications and require no special care in wearing or cleaning.
The best-known and generally least expensive variety in this group is carnelian. It ranges in color from yellow-orange to rich, near reddish orange, to orangey brown, and varies from semi-opaque to highly translucent. Iron is the source of its color and as a result it can be easily heat treated, (even by the sun’s heat alone,) to darken red tones as the iron is oxidized. You should assume, unless informed otherwise, that any piece of carnelian has been enhanced in this way. Most commercial carnelian comes from India, but it is mined world wide.
Apple green chalcedony that derives its color from nickel is chrysoprase. Ranging from nearly opaque to nearly transparent, its color spectrum includes olivey, to nearly pure greens of medium tone. Very fine, highly saturated pieces have been successfully misrepresented as Imperial jade. Most chrysoprase sold today comes from Australia. Prase is a darker, less saturated form, rarely seen, which comes from Eastern Europe. There are also very small amounts of a green chalcedony colored by chromium found in Africa, called Mtorolite.
Marketed as “Gem Silica” this relatively rare, blue to blue-green, opaque to near transparent material is the most expensive type of chalcedony. Found almost exclusively in Arizona its color is due to copper. Those who take the trouble to seek it out and are willing to pay the price are rewarded with a glorious color, (elsewhere found only in the soft gem Chrysocolla,) in a stone that has the durability and hardness of quartz.
This material is the darling of today’s gem carvers and jewelry designers. Piece after piece is featured in magazines like Lapidary Journal, Modern Jeweler, Metalsmith and Ornament. One look at the ethereal colors in this group will tell you why.
The various blues, each group of which has its vocal supporters, are generally designated by place names. They vary in depth of blue color and degree to which the blue is modified by gray or pink hues. As a group, they vary from pale to medium tones and in degree of translucency.
Some pieces have a slight adularescence that enhances their value. This phenomenon, which reaches its apex in moonstone, is due to light interference from layers of microscopic inclusions and looks 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, which nonetheless is called “Holly Blue.”
General high value points for all chalcedonies would be strong color saturation and high translucence. Sinkankas in his Standard Catalog of Gem Values, 2nd. Edition, and Federman in his Modern Jeweler columns discuss some price ranges for various high grade chalcedonies. Chrysoprase from $10-$30 per carat in finest grades, Gem Silica from $12-$50 per carat and blue chalcedony from $10 to $100 per piece. The IGS, (International Gem Society,) market price survey lists $10/ct as the going price for blues. Carnelian is common enough that its value is more a function of the beauty of the cutting than the material itself.
Turritella agate is composed mostly of turritella shells, embedded in agate.
Iris agate shows iridescent colors reflecting from between the color layers.
Sard is similar to carnelian, but with a brownish tone and more opaque.
Prase is a green, or yellowish green chalcedony.
Plasma is a dark green, opaque variety. It frequently has white or yellowish spots.
Bloodstone, or heliotrope, is plasma with red and orange spots of iron oxide.
Onyx is a chalcedony with straight bands of colors. Black onyx 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 are opaque, dull gray or white. They rarely make an appearance as gems, but are useful materials for arrowheads, driveways, and other utilitarian purposes.
Petrified wood and dinosaur bone are primarily chalcedony in their modern composition. The lapidary will cut and polish them like any other quartz family gem.
Text and photos courtesy of Barbara Smigel at Artistic Colored Stones.