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.
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, Onyx, Sardonyx, Binghamite, Bloodstone, Carnelian, Chrome-Chalcedony, Chrysocolla Chalcedony, Chrysoprase, Jasper, Dallasite, Orbicular Jasper, Plasma, Petrified Wood, Pietersite, Sard|
|Refractive Index||1.544 - 1.553|
|Polish Luster||Greasy to vitreous|
|Fracture Luster||Dull to waxy|
|Stone Sizes||To very large|
|Enhancements||May be dyed. Dying, 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. Others not diagnostic.|
|Identifying Characteristics||Dyed blue and green gems show pink in Chelsea filter.|
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.
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.
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.
- Agate is distinguished by transparency, or by having a pattern to its colors. Agate varieties are described by their color patterns, inclusions, or source.
- Bloodstone, or heliotrope, dark green, (plasma,) with red and orange spots of iron oxide.
- Carnelian orange to red
- Chrysoprase nickel colored green
- Chalcedony as a variety name, refers to a translucent material with a single color
- Gem Silica blend of chrysocolla and chalcedony
- Fire agate iridescence, brown chalcedony with a botryoidal growth form. It contains plate-like crystals of limonite in various planes, which create iridescence.
- Hawks eye chatoyant blue chalcedony. Related to tigers eye, but retaining its original color.
- Iris agate shows iridescent colors reflecting from between the color layers.
- Jasper refers to an opaque, solid colored stone usually stained by oxides. Variety names refer to color patterns, inclusions or source.
- Onyx is a chalcedony with straight bands of colors.
- Plasma opaque dark green, frequently with white or yellow spots
- Sard brownish tone and slightly translucent to opaque
- Sardonyx white and red layered onyx
- Tiger eye chatoyant brown or gold chalcedony.
- Turritella agate is composed mostly of turritella shells, embedded in agate
- Zebra tiger eye blue & gold chatoyant chalcedony
Source/Attribution: Barbara Smigel at Artistic Colored Stones.