Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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Chalcedony

Technically, chalcedony (kal SED' uh nee) is any form of cryptocrystalline quartz (meaning any form of quartz whose crystals are too small to be seen without extremely high magnification or a special type of microscope). It's a huge group of gem materials including translucent, single color types, banded & patterned translucent types (agates) and opaque patterned and single color types (jaspers).

In common practice, however, only some of the translucent, single color types are sold as "chalcedony" whereas the rest of this group is sold under individual variety names, or as jasper or agate. In this essay chalcedony will mean any translucent, cryptocrystalline quartz with a single color, whether it has a special varietal name or not. Formed either through hydrothermal processes or through evaporation/percolation they often occur as seams, between rock layers, or in pockets within other rocks. As percolation of dissolved silica solution occurs through buried organic material, a type of fossilization, known as petrifaction, can occur, leaving a stone replica (pseudomorph) of the orignal bone, shell or wood. It is not unusual for the surface of chalcedony to be botryoidal.

[Typical appearance of chalcedony rough, in this case chrysoprase, chalcedony geode, petrified pine cone]

The various types differ in color due to metallic impurities, such as iron, nickel, copper and titanium which are present during crystallization, or to internal structure or inclusions. Pieces may also have inclusions that create interest.

[Most commonly known color varieties of chalcedony: carnelian (iron), chrysoprase (nickel), blue chalcedony (light interference from inclusions and internal structure)"gem silica" (copper), chalcedony from Indonesia with iron staining, dendrites of manganese oxide in chalcedony]

[Unusual color forms of chalcedony: grey-green Mtorolite (chrome) Pink-Orange (manganese), near colorless]

This group of stones is almost always either 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.

Carnelian: The most well 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. Carnelian is the only type of chalcedony which is routinely enhanced. 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 carnelian comes from India.

Chrysoprase: 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 tones. 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.

Chrysocolla Chalcedony: 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.

Blue Chalcedony: This material is the darling of today's gem carvers and jewelry designers. Piece after piece is featured in magazines like Jewelry Artist, 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 grey or pink hues. As a group they vary from pale to medium tones and in degree of translucency. Some pieces have a slight adularescence which 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 Washington, respectively and are slightly to moderately greyish blue with a light to medium color range. Blue chalcedony from Namibia, often called African Blue, varies from greyish 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".

[Blue Chalcedony bracelet with numerous Holly blue stones, some of which are showing adularescence, botryoidal blue chalcedony from Africa]

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Value

General high value points for all chalcedonies would be strong color saturation and high translucence. Custom cuts and large size add largely to value with chrysocolla chalcedony, fine chrysoprase and Holly Blue being, in general, the most valuable types. Carnelian is common enough that its value is more often a function of the beauty of the cutting than the material itself.

Gemological Data:

Makeup: Silicon dioxide, in cryptocrystalline form

Luster: vitreous

Hardness: 7

Crystal structure: trigonal

Cleavage: none

Density: 2.60 (+.10, -.05)

RI: 1.53-1.54

Birefringence: none to .004

Pleochroism: none

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