Chalcedony

Chalcedony




Chalcedony, or cryptocrystalline quartz, is a unique mineral with a fascinating history. Many of its colorful varieties were highly esteemed by early peoples, both for personal adornment and utilitarian articles. Today, even though it is found in abundance throughout the word and is one of the least expensive gem materials, it enjoys a greater popularity than ever before. The seemingly endless array of colors and patterns of color distribution in which chalcedony occurs presents a never ending source of pleasure and a constant challenge, to the amateur lapidary and "rock hound".

The origin of the world chalcedony (pronounced kal-SED-oh-nee) is uncertain. The obvious belief is that it came from "Chalcedon" a sea ports in Asia Minor near Byzantium, and a probable early source. The Greek word is "chalkedon", in Latin it is "chalcedonius" The use of the modern spelling can be traced only to the early part of the sixteenth century.

Most of the chalcedony varieties have an intriguing historical background and more than the usual share of legends, superstitions and strange beliefs. Agate was one of the first gem materials known to man. According to legend, it made the wearer agreeable and persuasive and gave him the favor of God; too, it assured him of victory and strength.

It was supposed to cure insomnia and make the owner have pleasant dreams. It was also said to assist the owner in acquiring riches by making him more cautious and prudent. The Sumerians, the earliest inhabitants of Mesopotamia, seem to have been the first to use agate and other forms of chalcedonic quartz for seals, signet rings, beads and other articles of jewelry. Since the Sumerian culture apparently was well developed when the Egyptian was in its infancy, it can be seen that chalcedony was used and cherished from earliest civilization. The Sumerians also used agate for making ceremonial axe heads, an excellent example of which is in the American Museum of Natural History in New York City. An engraved inscription sets the date of the instrument at between 3000 and 2300 B.C.. In early times, agate was a favorite material for carving into bowls, bottles and cups. An excellent example, cut from a single piece of agate, measures twenty eight and one-half inches in diameter. This piece was taken as loot by the Crusaders and carried to Europe; it is now preserved in Vienna. What is often said to be the most outstanding object fashioned from agate is a two-handled wine cup, with a capacity of more than one pint, intricately carved on the outside with Bacchanalian subjects. Historical records tell us that it was made for the Emperor Nero, and that, after being owned by many persons, was give in the ninth century by Charles the Bold to the Abbey of St. Denis, where it was used for centuries to hold the wine at the coronation of the kings of France. This cup is still, considered by the French to be one of the most valuable mementos of their history. The Persians, Arabs and other oriental peoples had great regard for the various types of agate, undoubtedly because of its color and durability. They used it principally for finger rings, upon which was carved a verse from the Koran (in more recent periods), the owner's name, or some magical or symbolic figure. Such rings were, and still are, considered powerful talismans, protecting the owner from a wide variety of calamities. The name agate comes through the Latin from a Greek word for the river Achates, in Sicily, where the material was first found in quantity in the time of Theophrastus. Mocha stone, another name for moss agate, is derived from a port on the Red sea coast of Arabia called Mokha, once famous but now little used.

The word chrysoprase comes from the Greek meaning "golden green". One of the superstitions associated with this yellowish-green variety states that if a thief sentenced to death holds a chrysoprase in his mouth he will escape his fate. Although the legend does not describe how this miracle was accomplished, historical records indicate that the stone was believed to make the prisoner invisible. Another interesting story concerns a chrysoprase that Alexander the Great wore in his girdle during all of his battles. According to this fable, he laid aside his girdle to bathe in the Euphrates River after his Indian campaign and a serpent bit off the stone and dropped it into the river.

Carnelian, the original but now obsolete spelling of carnelian, was derived from the old Latin word "cornum" meaning "cornel berry" or "cornelian cherry". The newer spelling came into existence during the fifteenth century, as a result of the mistaken belief that it originated from the Latin word "carneolus", after "carnem" meaning "flesh". Legend says that this red to brownish-red to orange-red variety had the power to drive away evil and bring good luck. It was supposedly the stone for weak or timid voices, giving courage to speak boldly and well. It protected against the envious and was responsible for making sure that desires were gratified. The popularity of carnelian among Mohammedan people may have been due to the fact that Mohammed himself wore one as a signet ring.

Sard, which is similar to carnelian but darker and more brownish, comes through the Latin from the Greek word "Sardis", which was the capital of Lydia in Asia Minor. Sardonyx, a name that describes stones consisting of bands of sard and black and white levers, alternates with peridot as the birthstone for the month of August. Sard was long thought to be a protection against incantations and sorcery and was believed to sharpen the whites, rendering the wearer fearless, victorious end happy. The color of this stone was supposed to neutralize the malign influence of the dark onyx, driving away the bad dreams caused by the latter and dispelling the melancholy thoughts inspired.

Bloodstone, or heliotrope (HEEL-ee-oh-trope), the dark green chalcedony spotted with red, alternates with aquamarine as the March birthstone. Heliotrope comes from two Greek words meaning "sun" and "direction". Bloodstone was once used as an amulet in the belief that it would stop bleeding. As with many of the chalcedony varieties, bloodstone has been used as a gemstone since earliest recorded history.

According to legend, it could cause the sun to turn red and make thunder and lightning occur. It was supposed to give the wearer clairvoyance and preserve his faculties and health. It meant respect to the wearer and guarded him from deception. In addition, he was supposed to be given what he asked for and to be believed, regardless of what he said. All doors were open to the owner of a carved bloodstone.

Varieties of Chalcedony

Since quartz is the most abundant mineral in the earth's crust and the minutely crystalline chalcedonic form is found in every country, it is not surprising that there are a large number of distinct varieties. Although hobbyist rock and mineral collectors distinguish literally hundreds of varieties of chalcedony, those described below are the only ones of significance to the jeweler.

  1. Chalcedony

    In general usage, the terms chalcedony and cryptocrystalline quartz are synonymous; therefore, chalcedony may be applied correctly to any of the numerous varieties of this large mineral family. However, it is also sometimes used as a specific variety name to refer to the semi-transparent to translucent white to pale grayish-blue material.
  2. Chalcedony "Moonstone "

    This variety is semitransparent white to gray (milky) chalcedony that only vaguely resembles moonstone. It lacks the floating light effect that is characteristic of true adularescence in genuine moonstone. Buy moonstone necklaces
  3. Chrysoprase

    Semitransparent to translucent light to medium yellowish-green chalcedony is called chrysoprase. This term should be reserved for the natural material; it should not be applied to the dyed dark green chalcedonic quartz that is often sold as either "green onyx" or "chrysoprase"
  4. Carnelian

    Semitransparent to translucent red, orange-red to brownish-red or brownish-orange chalcedony is called carnelian. These colors may also be produced by heat treating nearly colorless chalcedony. Buy carnelian necklaces
  5. Sard

    Sard is similar to carnelian, except that its color is usually less intense; i.e., more brownish and somewhat darker. It tends more to the dark-reddish colors and carnelian to the lighter, predominantly orange hues. Unfortunately, there is no distinct dividing line between sard and carnelian.
  6. Bloodstone or Heliotrope

    Semi translucent to opaque dark-green chalcedony with red to brownish-red spots is called either bloodstone or heliotrope. It is often used for men's rings, either as flat tablets or in carved forms. Buy Bloodstone necklaces
  7. Prase

    The term prase is applied to translucent light-grayish yellow-green chalcedony. It finds little application in the jewelry trade.
  8. Agate

    The term agate is applied correctly to chalcedony in which the color is distributed in curved bands or layers. The banded appearance is distinguished by a difference in either color or translucency, or both, between adjoining layers of chalcedony. The word agate is often used somewhat loosely with a prefix to describe material that is not banded; e.g. landscape agate (containing inclusions resembling a landscape), fortification agate (composed of straight, intersecting bands), and moss agate or mocha stone (milky-white chalcedony with green, black or brown inclusions distributed in dendritic patterns). Because of its rather porous nature, gray and white agate is particularly susceptible to dyes of various colors. The dye may color the material unequally, preserving the banded appearance, or it may penetrate porous materials so evenly that a single color results. Buy agate necklaces
  9. Onyx

    The only proper use of the term onyx is to describe chalcedony composed of straight, parallel bands. It is used incorrectly when applied to grayish chalcedony or agate that has been dyed to produce the solid-colored material known e s "black onyx", "green onyx", etc. These misnomers have been used for this purpose for so long and so consistently that many jewelers do not realize that they are incorrect and misleading. If the onyx exhibits bands of sard colors alternating with either white or black parallel bands, it is known as sardonyx. Cornelian onyx has alternate parallel bands of carnelian colors with either black or white. White banding is much more common than black; in fact, natural black chalcedony is not too frequently encountered. A cross-section of chalcedony showing both curved bands (agate) and straight, parallel bands (onyx).
  10. Jasper

    The name jasper is applied to most of the semi translucent to opaque chalcedonies that occur throughout the word in a wide variety of colors, often without a pattern of any kind. It may be red, yellow, brown, green, grayish blue or any combination thereof. It is frequently dyed blue and sold as "Swiss lapis" or "German lapis".
  11. Plasma

    Semi translucent to almost opaque dark-green chalcedony with white or yellowish spots is called plasma. It is essentially bloodstone but lacks the red spots. Both plasma and prase are sometimes used as inexpensive jade substitutes.
  12. Chrysocolla Chalcedony

    Translucent to semi-translucent intense light-blue or blue-green chalcedony, the color of which is caused by minutely distributed chrysocolla (a copper silicate), is one of the loveliest varieties of chalcedony. In its finest quality it rivals the most beautiful turquoise in appearance. Unfortunately, however, it is quite rare.
  13. Agatized Wood

    Wood that has been replaced entirely by chalcedony resembles a patterned jasper, and is a rather attractive material for ornamental purposes. Almost all chalcedonic colors are represented, including red, yellow, black and brown. It is also called silicified or petrified wood.

Formation & Sources of Chalcedony

Cryptocrystalline quartz is deposited at relatively low temperatures by circulating ground waters or by magmatic waters that have dropped to temperatures approaching those expected at the earth's surface. The earth's crust is so rich in silica that ground waters commonly approach saturation in silica content. As a result, chalcedony or other forms of quartz are often deposited in cavities in the near-surface rocks. Such waters may dissolve away pre-existing materials such as wood, bone and shells and replace them particle by particle with chalcedony. Chalcedony and/or crystalline quartz may fill or line cavities of any kind in near-surface rocks to form concretions or geodes.

Sources and Recovery of chalcedony

Because chalcedony is distributed widely throughout the world, any attempt to list and describe in detail all sources would be impractical in a work of this nature. Listed below, however, are some of the more notable localities.

White to pale grayish-blue chalcedony:- Siberia, Ice-land, India and the beach and desert regions of California.

Chrysoprase:- Australia, Silesia, California, Oregon, and India.

Moss agate:- India, Wyoming, Montana, Washington, Oregon, Texas and most western states.

Chrysocolla chalcedony:- Arizona, New Mexico and the Lake Superior region.

Carnelian and sard:- Brazil, India and Uruguay.

Bloodstone:- India, Siberia, Australia, Scotland and Brazil.

Prase:- Germany.

Plasma:- India, China and Germany.

Banded agate and onyx:- Brazil, Madagascar and Uruguay.

Jasper:- Germany, Scotland, Sicily, Siberia, California, Oregon, Idaho, Texas, Montana and most western states.

Agatized woods:- Arizona, New. Mexico, California, Washington, Oregon and most western states.

Although chalcedony has the durability to make it common in alluvial deposits, profitable mining is usually confined to primary deposits. The low price of most varieties precludes the practicability of sorting gem gravels to select satisfactory pebbles. Since the bulk of the material is heated or dyed to improve color, masses of cheap gray agate are blasted out of extensive deposits for this purpose.

Uses of Chalcedony in Jewelry & Fashion

Chalcedony is fashioned in a wide variety of shapes and forms, including beads, carved and engraved gems, pen stands, figurines, spheres, bowls and every variation of the cabochon cut. The larger piece, of agatized wood, agate and jasper are particularly popular for bookends, paper weights and similar objects. The majority of agates for commercial consumption are stained and cut at Idar-Oberstein, Germany. Large quantities of almost all chalcedony varieties are tumbled for use in earrings, dangle bracelets, pins and many other-jewelry articles.

Since chalcedony is one of the tougher gemstones, cutting does not present any of the usual problems tor the lapidary. Orientation is necessary only to obtain the desired pattern (scenes, flowers, trees, etc., in agate), or for color or fractures. It is not overly affected by heat, and only the usual care in dopping, grinding and sanding is necessary. Chalcedony is one of the easiest gemstones to fashion; hence, it is widely used in introducing the novice to the lapidary arts. It is usually polished on a felt lap, using tin oxide or cerium oxide as the polishing agent. A few of the jaspers that tend to undercut should be polished with Linde A or chromic oxide on a leather lap.

Because the Idar-Oberstein district is a world famous agate cutting center, a brief description of its activities and history is appropriate in this assignment. Idar and Oberstein, which are twin towns in the Idar Valley of the Nahe River in Germany's lower Saar Basin, have been noted for their lapidary craftsmen for almost two thousand years. One of the sights of Oberstein, which has a population of about 20,000, is an ancient church high up on a cliff above the river that is partly carved out of an old agate bearing lava flow. Idar, with about 12,000 inhabitants, forms that part of the twin town that lies in the steep-walled tributary of Idar Brook. The surrounding hills consist of old basaltic lava, in the steam cavities of which occur numerous concentrations of agate, crystalline quartz and calcite.

Legend and history have it that the industry had its inception in Roman times, when Caesar was pressing northward after his conquest of Gaul. It is said that in those days a troop of Roman soldiers crossed a mountain range and stumbled upon a small and unpretentious group of dwellings perched on precipitous walls of rock between which ran a rapidly flowing stream. The native presents and hunters all had collections of beautifully marked stones that they had broken from the rocks surroundings their village. Among these collections, the Romans, the Romans recognized the same materials from which the artisans and goldsmiths of the Imperial City cut and polished the precious stones that were so prevalently worn at that time. Thus began a period of lively activity in the little settlement, which was then known as Hidera. The water power of Idar Brook was harnessed and great grindstones were installed. By the year 31 A.D., Idar was an important gem-cutting center.

Although demand diminished with the fall of the Roman Empire, the inhabitants never ceased their activity in the handicraft that had been introduced to them. During the twelfth century, the Counts of Oberstein, among other German lords who owned the lands and the quarries, controlled the important business of supplying gems to adorn the armor and swords of the knights and the attire of the castle maidens. The fifteenth century saw a well-established lapidary industry that was devoted entirely to cutting the material mined from local deposits. A cutters guild was organized in 1609 that included three specific working groups; the stone pickers, the grinders and the drillers. Soon the fame of Idar-Oberstein and its superior craftsmen began to spread, and many foreign buyers came to the Idar Valley. In the early nineteenth century, when the supply of rough material became almost nonexistent, the industry managed to continue on a reduced scale by using Swiss rock crystal and smoky quartz, Saxon and Bohemian amethyst, and Indian carnelian, moss agate and jasper. In 1827 the Uruguay-Brazil agate fields were discovered by emigrants from the Idar district, and the importation of this material revived the languishing industry.

Today, the cutting works are located not only in the twin towns but in the surrounding villages and farms as well. The cutting of agate offers a supplementary source of income to farmers, woodcutters, and others during winter months and other periods of slack time. In past years, the numerous mountain streams furnished waterpower for the small mills along their banks. Electrically driven machinery has replaced the more picturesque waterwheels. The old fashioned manner of agate cutting so frequently illustrated in descriptions of Idar-Oberstein, which required that the cutter lie prone before a large sands tone-wheel, is now obsolete. Although Idar was fortunate in being saved from destruction during World War II, the industry suffered greatly as a result of lack of material and import and export difficulties. Since that time (however, the two towns, recognizing the historic interest of the industry and their commercial dependence on it, have taken steps to foster it. Schools to teach gemology and the lapidary arts. have been established to train new workers. Many craftsmen, as a means of bolstering the economic structured of the community, are now applying their talents to diamond cutting and the shaping of agate and synthetics into balance stones, mortars and pestles, bearings and many other kinds of scientific and technical apparatus. As a result of these and other constructive efforts , Idar-Oberstein is now more prosperous than ever before.

Physical & Optical Properties of Chalcedony

Physical

Chemical Composition Silicon dioxide, or silica, expressed by the formula SiO2. The same composition as crystalline quartz, except that it usually contains larger amounts of impurities and may contain some amorphous silica in the form of opal.
Crystallographic Character Cryptocrystalline; i.e. a compact, homogenous mass of submicroscopic doubly refractive crystals. A fibrous kind of crystalline structure (but not amorphous, for the most part).
Hardness 6½ to 7
Toughness Good; about the same as crystalline quartz.
Cleavage None
Fracture Conchoidal
Specific Gravity 2.55 to 2.65 , normal: 2.60
Streak White
Characteristic Inclusions Moss agate contains dendritic (treelike) inclusions of manganese oxide or sometimes iron oxide. These inclusions, which resemble trees or plants, may be black, green, reddish, yellowish or brownish. Segenite agate contains inclusions of actionolite, hornblende, rutile or other needlelike crystals.

Optical

Degree of Transparency Semitransparent to opaque. There are no transparent varieties.
Luster Dull to waxy on rough surfaces; greasy to vitreous on polished stones.
Refractive Index 1.535 - 1.539 (both readings are sometimes evident using a monochromatic light source)
Birefringence Weak
Optic Character Since chalcedony is a crystalline aggregate its optical characteristics cannot be determined by the usual gem testing methods. In the polariscope, translucent stones remain light during rotation.
Pleochroism None
Dispersion None
Phenomena Iris agate exhibits a display of rainbow colors (usually red, green and blue) caused by diffraction of light from a multitude of very fine growth lines or bands. The so-called fire agate exhibits rainbow colors as a result of interference of light from exceedingly thin layers of iron oxide on the deeper layers of the stone. Both of these are rare.
X-Ray, Fluorescence All varieties are generally inert under both long and short wavelengths. Some, however, may fluoresce a weak Green or blue, especially uranium bearing materials from Montana and Australian chrysoprase.
Color-Filler Reaction Dyed except dyed green may show reddish.
Absorption Spectra Dyed green chalcedony may show vague chromium lines centered at about:7050, 6700 and 6450 A.U.

Effects Caused by:

Heat Infusible before the blowpipe, but the color may be affected.
Acids Attacked only by hydrofluoric acid. Nitric acid will attack aniline dyes.
Irradiation No data available.

Test and Identification of Chalcedony

Chalcedonic quartz, in one form or another, can be confused with almost every translucent to opaque gemstone. Nephrite, jadeite, malachite, lapis Lazuli, moonstone, amazonite, turquoise, shell and many other materials are closely duplicated in appearance by one or more varieties of natural or dyed chalcedony.

Green chrysoprase and green-dyed chalcedony can be separated from amazonite by the appearance of minute fractures along the girdle edge. Amazonite displays cleavage, whereas chalcedony shows only its characteristic dull to waxy conchoidal fracture. In addition, amazonite has a characteristics appearance; a sort of shiny reflection that appears to be just beneath the surface, accompanied by a grid like color distribution. Chalcedony, on the other hand, is usually rather evenly colored and lacks both of the characteristics mentioned for amazonite. They can also be distinguished by a careful spot reading on the refractometer, for amazonite has slightly lower indices. A heavy liquid set at 2.57 or 2.58 also separates the two stones.

Chrysocolla chalcedony, which in its finest quality bears a close resemblance to high-quality turquoise, can be distinguished there from by physical properties. Chrysocolla chalcedony is considerably less dense (2.60 to 2.75 plus) and has a lower R.I. (1.535 compared to 1.61). In appearance they are almost identical, sharing the translucency expected of the finest turquoise. It is impossible to distinguish between these two minerals on the basis of fracture or fracture luster, since both have a conchoidal fracture and a dull to waxy luster on fracture surfaces.

The mineral chrysocolla, which is distinct from chrysocolla chalcedony, has a blue enamel like appearance, in a color similar to that of turquoise, but it is soft (2-4) and crumbles easily. The refractive indices have been reported from about 1.4 to nearly 1.6, but 1.464 to 1.57 is usual. The S.G. is 2.2 to 2.4. Really, its only function in the gem field is to act as a coloring agent for chalcedony.

Dyed jasper made to imitate lapis lazuli is seldom a highly effective imitation on the close inspection. True lapis usually contains flecks of pyrite, whereas the dyed jasper does not. In addition, the jasper will give a characteristic chalcedony R.I. of approximately 1.535, as opposed to approximately 1.50 for lapis.

Jadeite and nephrite are distinguished from chalcedony readily by either refractive index or specific gravity test, since both jades have much higher property values.

Although dyed green agate may resemble malachite superficially, the very high S.G. and R.I. of malachite serve to separate it readily. To one who is familiar with the appearance of both materials, there is little resemblance. Malachite often displays a radial fibrous structure that is never obvious in agate. The two colors in malachite are very dark green and a lighter green, whereas one color in agate is usually white or almost white.

One of the key characteristics of chalcedony in its translucent to semi translucent forms is its behavior in the polariscope; since it is composed of a multitude of minute crystals with a random orientation, it remains tight in all positions in this instrument, rather than changing from light to dark. Moonstone, for example, shows extinction upon rotation, and in some positions exhibits an interference figure. Another characteristic of some translucent chalcedony is the fact that in certain lights it is possible to see a rather shadowy structure, which would suggest that the material is composed of rounded "blobs" of a fraction of a millimeter to about one centimeter is size.

Most stained chalcedony has a very intense color that is not characteristic of the natural. With the exception of chrysocolla quartz, the color intensity of unstained material is rather subdued. Moreover, the color of such attractive atones as chrysoprase is usually some what mottled, in contrast to the solid, even green of the more intensely colored stained green chalcedony.

Chalcedony is often imitated in glass. Usually, a separation can be effected by observing the luster of fracture surfaces; that of glass is vitreous, as opposed to the dull to waxy luster of chalcedony. In addition, semi transparent to translucent glass often shows bubbles and swirl marks close to the surface. The dendritic patterns in glass imitations of moss agate are much too uniform for this substitute to be confused with the natural stone.

Shell cameos are sometimes confused with stone cameos, especially those carved from carnelian and white-layered agate or onyx. These may be distinguished readily by applying a tiny drop of hydrochloric acid, for the shell effervesces strongly and chalcedony fails to react. Shell's low (3-4) hardness is also easily detected.

Opal without play of color may closely resemble chalcedony of comparable color. A spot R.I. reading shows about 1.45 for opal and 1.53 for chalcedony. Translucent opal may well remain light in all positions in the polariscope; therefore, that instrument does not assist in this separation, unless the stone remains dark. S.G. differs widely, also.

Buying Tips for Chalcedony

The chalcedony group of gem varieties offers the consumer an opportunity to "have his cake and eat it too". These stones, although lovely, are so plentiful that many are priced at a figure less than synthetic substitutes for other gemstones. In chalcedony, we see an excellent example of missed opportunities for jewelry sales. One of America's fastest growing hobbies is collecting gem materials suitable for cabochon cutting. The great demand for chalcedony for such purposes is proof of the beauty that people see in it. It is so inexpensive when cut abroad that retailer could obtain a large number of stones at a very low price to feature in window displays and call attention to its attractiveness for use in cufflinks, tie clasps and also many items of inexpensive jewelry for women.

Because of the great quantities of chalcedony that are available, the jeweler should be particular in choosing specimens for stock. It is frequently possible to purchase good quality for just a few cents more than poor quality. Since the finer grades are much more attractive and thus more saleable, it is wise to spend the additional few cents to obtain stones that will "move".