Peridot


Peridot

Olivine, Chrysolite, peridot, topaz, forsterite, fayalite, "evening emerald" and sinhalite are all terms that have been concerned with the stone discussed in this assignment. First, the gem is pronounced PEAR -uh-doe, not PEAR-uh-dot. The mineralogist calls it OLIVINE (pronounced AHL-uh-veen), although he calls an olivine-rich rock PERIDOTITE (pear-ID-oh-tite). On the other hand, if the rock is all olivine, he calls it DUNITE (DEW-nite). The term peridot comes directly from the French "peridot" which in old French was "peritot". The origin of the word is uncertain, however, the form "perldota" was in use in England in the thirteenth century and later it was called "peridote"

The word CHRYSOLITE (KRIS-oh-lite), from the Greek words for flgodlenfl and "stone". It was used by Pliny for topaz-quartz and possibly topaz: today, however, it is often applied to the greenish-yellow to yellowish-green peridot. Further evidence of the lack of logic and consistency of present day terms is the rather clear evidence that Pliny's so called topaz ("topaziusff) referred to our peridot, for he mentioned that it came from an island in the Red Sea, the chief source of gem quality peridot. FORSTERITE and FAYALITE are mineralogical terms for close relatives of peridot, and "evening emerald" is a relic of the period when any gem less expensive than emerald, ruby, sapphire or diamond was given a fanciful name involving another gemstone. SINHALITE (SIN-ha-lite) is a newly described mineral long thought to be a variety of peridot.

On the basis of all the terms connected with this gem over the centuries, a plausible argument could be made for calling it almost anything. However, peridot has a pleasing sound when pronounced correctly. Chrysolite is too reminiscent of the quartz variety, and the sound of the word olivine has little appeal; for many years, however, it was ( and still is, too often) applied to demantoid garnet.

In common with many other gemstones, peridot has an interesting background of legend and superstition. It was called "the gem of the sun" by the ancients. Their belief that it had the power to dissolve enchantments and to put evil spirits to flight was probably due to this association with the sun, before whose life-giving rays darkness and all the powers of darkness were driven away. For peridot to exert its full powers as a tailsman, it had to be set in gold; worn in this way, it was thought to dispel the terrors of the night, however, it were to be used as a protection from the wiles of evil spirits, the stone had to be pierced and strung on the hair of a donkey and then attached to the left arm. When powdered, peridot was used as a remedy for asthma; when held under the tongue, it was believed to lessen the thirst in fever.

Many beautiful examples of peridot were brought back from the Mediterranean area during the Crusades and presented as emeralds to European cathedrals, where they still remain. One of the finest examples is among the jewels that formerly belonged to the Russian czars. Now in the Diamond Treasury in Moscow, it is a lovely yellowish-green stone, weighs 192.75 carats, and is almost flawless. In bygone days, it was mounted in a handsome setting surrounded by thirty diamond brilliants. Today, peridot alternates with sardonyx as the birthstone for August.

Varieties And Trade Names

The two old terms frequently applied to peridot (i.e., olivine and chrysolite) outline its color range fairly well, for chrysolite means "golden yellow" and olivine has reference to its olive-green color. Actually, transparent peridot varies in color from a light greenish yellow through medium olive green to a rather dark brownish green, although there has been some tendency to categorize trade grades as follows:

  1. Peridot
    This term is applied to the top grades of this gemstone. It is a medium to dark, slightly yellowish green, which to many gives the impression of a swarm, velvety color. It is never as intense in color as the most expensive grades of emerald or demantoid garnet, but to those who appreciate peridot, this is an important facet of its beauty.
  2. Chrysolite
    The color range of this type is light yellow-green to greenish yellow.
  3. Olivine
    The term olivine refers to dark yellow-green to brownish green to almost brown stones.

The only term in general use is peridot. Actually, this is the only quality for which a significant demand exists; but even when lighten or darker material appears on the market, it is rarely offered other than as peridot. The term chrysolite is very rarely encountered. When olivine is used, it refers more often erroneously of demantoid garnet than correctly to a grade of peridot.

Peridot is actually an intermediate member of a group of minerals varying from forstertte, a pure magnesium silicate, to fayalite, a pure iron silicate. With no iron whatever, forsterite would be colorless, however, in nature, it is never totally free of iron; therefore, it is at least yellow in color and usually yellow-green. Fayalite, on the other hand, is a dark-green to brown or black stone. Gem peridot is much closer to the magnesium-forsterite end than to the iron-fayalite end of the series.

Formation of Peridot

Peridot (or olivine, as the mineralogist calls it) is a mineral that is very common in nature, particularly in basic igneous rocks (i.e., those low in silica content). It is so common that a major igneous rock type is called peridotite. However, gem peridot is very rare.

Slnce diamond-bearing kimberlite is a type of peridotite, peridot, as might to be expected; is an important constituent of the peridot has altered to serpentine, peridot is an important constituent of kimberlite in the lower reaches of the mines. However, it does not occur in large enough fragments to be of interest as gemstones.

In some areas of the world, rocks made up almost entirely of peridot are found. Most of the major chromium deposits in the world occur with the mineral chromite disseminated in a rock called dunite. Although the grains of peridot in dunite are transparent and of a lovely color, they are too small to be of interest as gemstones. Only very rarely do igneous rocks in which peridot occurs have peridot crystals of a size and perfection to be of interest to the gemologist. Since olivine is one of the earliest minerals to crystallize from an igneous melt, it often occurs as fairly large crystals in a minutely crystalline groundmass; however, it is usually too badly fractured to provide gem crystals. Thus, despite the wide distribution of this rock, it is exceedingly rare when the major constituent is found in gem quality. In addition, where it is formed in gem quality crystals, its tendency to alter rapidly to serpentine reduces further the chances of finding crystals of gem quality.

The two situations in which gem quality crystals seem to occur are in cavities in an extrusive igneous rock and in contact metamorphism of sedimentary rocks containing magnesia and silica, such as impure limestone or dolomite.

Recent studies by Professor Richard Jahns, Ph.D., indicates that large gem quality crystals are unlikely to occur in magma that cooled under ordinary conditions at depth. He has demonstrated that an essential condition to the formation of large crystals of excellent quality, structurally, is for themelt to reach a condition of water or other fluid saturation. Such a condition could occur quickly, if the magma moved toward the surface with an accompanying reduction in pressure. With a highly volatile liquid or gaseous phase, large, fine crystals may grow at very rapid rates (in a matter of weeks or months). Of academic interest only is the fact that peridot is sometimes found in meteorites. The grains are always very small and never of gem quality.

Sources of Peridot

There has probably been as much mystery connected with the sources of gem quality peridot as with anyone of the other gemstones. Many mineralogical and gemological textbooks have been written in which vague references to unknown sources in the Middle East are made. There is little doubt that over the years the most important source of gem peridot has been the island in the Red Sea known variously as Zeberged, Zeberjed, Zebirget or St. John's. (Note: Zeberget is the Arabic word for peridot.) This is undoubtedly the island in the Red Sea referred to by Pliny as being the peridot source of the ancients. It is slightly north of the Tropic of Cancer about fifty miles from the Egyptian port of Berrenice. Here beautiful medium-dark-green crystals are found in cavities in an altered peridotite.

Historical records show that the Red Sea deposits were worked as early as 1500 B.C. In those early days of mining Zebirget was known as the "Isle of Serpents", because it was infected with poisonous snakes that seriously impeded the recovery opera lions.

Later, however, the reigning Egyptian king had the pests eradicated so that the production could be continued without interruption. The workers were the inhabitants of the island, who were forced by the king to dig for the stones and deliver them to the royal gem cutters for fashioning. The stones were so highly prized by the monarchs of that time and so fearful were they of theft, that guards were posted with orders t0 put to death any suspicious persons who approached the shores. Only provision boats and official representatives of the king were allowed to land with impunity. Writers of the Peridot state that the visits of the provision boats were sporadic, and that this, combined with the heat and poor working conditions in general, made the lot of the miners an unhappy one indeed. According to legend, even those who had the right to seek the gems could not see them in daylight, because it was difficult to distinguish such bright, lustrous, it is said that prospecting was undertaken only after nightfall, when the searchers were able to detect the location of the stones by their radiance. They would mark the spot and return the next day to work the area. Production from St. John's has always been very sporadic, especially in the twentieth century. Evidently, the deposit is mined only after existing stocks have long since vanished. Prior to world war II there was an extend period when no large, fine quality material was readily available.

Shortly after the war, however, the quantity sufficient to stock colored-stone dealers very adequately entered the market but apparently there was never enough immediately available to depress prices. The second major source from which a few stones have come over the years in the Bernardino Valley in the Mogok area of Upper Burma. The crystals from this locality are often frosted and a slightly lighter green than the usual St. John's material. Burma has produced some sizeable stones within the last decade.

Excellent light-green stones, locally called "Job's tears", are found from time to time on a Navajo Indian Reservation in Arizona, where they are picked up as small pebbles on the surface. About the turn of the century, a West Coast gem dealer hired a number of Indians to gather Peridot for him and look several thousand carats to Paris to sell. He obtained an excellent price for the small quantity he offered at first. Later, however, he made the mistake of disclosing the large quantity he had on hand, irretrievable reducing the desirability, and thus the price, of what he had to offer.

Other sources of gem Peridot in small quantities are Minas Geraes, Brazil; Queensland, Australia; and Czechoslovakia. It also has been reported from Norway. Hawaii and the Belgain Congo.

Physical & Optical Properties of Peridot

Physical Properties
Chemical Composition A silicate of magnesium and Iron, expressed by the formula (Mg, Fe)2 SiO4 Crystallographic.
Crystallographic Character Orthorhombic system. The crystal habit is prismatic, often flattened. Well formed crystals are rare.
Hardness 6½ to 7
Toughness Good;
Cleavage Fairly distinct
Fracture Conchoidal
Specific Gravity 3.32 to 3.48. In green stones of gem quality the usual range is 3.32 to 3.35.
Streak White in green stones of gem quality; Yellowish in very dark-colored types.
Characteristic Inclusions Round, flat (dislike) areas of liquid and gas inclusions, usually with a small, opaque black inclusions "near" the centre. Lilly pad inclusion.

Optical Properties
Degree of Transparency Transparent
Luster Polished surfaces are vitreous; fracture surfaces are vitreous to sub vitreous.
Refractive Index 1.654 to 1.690. Gem-quality material shows little variation from these minimum and maximum readings.
Birefringence 0.036
Optic Character Either biaxial positive or negative. Varieties containing excessive iron (very dark colored) are negative.
Pleochroism Weak yellow-green and green. Distinct only in dark stones.
Dispersion 0.020
Phenomena None.
X-Ray, Fluorescence None.
Color-Filler Reaction Green.
Absorption Spectra Three strong, narrow bands in the blue at 4970, 4740 and 4536 A.U.

Effects Caused by:
Heat The blowpipe whitens but does not fuse the lighter or more yellowish colors. Dark-green colors fuse. Uneven or rapid heal may cause additional fracturing or complete breakage of the stone. Extreme heat must be avoided in cleaning, repairing, mounting and cutting.
Acids Attacked easily by Sulphuric acid and less easily by Hydrochloric acid.
Irradiation No effect.

Test and Identification of Peridot

Separating Peridot from substitutes or imitations or fakes should be relatively simple, despite the fact that many materials occur in a green color that may resemble Peridot colors. Peridot has a number of distinctive characteristics. The combination of the great doubling of the back facets visible under magnification and the wide separation of the refractive-index readings identifies Peridot almost without further testing.

Most gem Peridot has an angle between its two directions of single refraction that is close to 90°. As a result the middle index for this biaxial stone is usually almost exactly half way between the highest and lowest readings. If as a Peridot is rotated on the hemisphere of the refractometer, a single reading is seen at any point upon rotating it is usually just above 1.67, whereas the extreme will be near 1.654 and 1.690. Peridot has a specific gravity very nearly equal to that of the heavy liquid methylene iodide (3.32). In most instances, it sinks very slowly in that liquid. Peridot never displays strong dicroism; in fact. it is usually very weak, with one direction yellow-green and the other green.

Materials that may resemble Peridot in appearance include glass (Peridot is often referred to as having a "bottle-green" color), tourmaline, assembled stones and green zircon. Of these, only glass has an index close to that of Peridot, and it is separated readily by its single refraction. If only the numerically higher of the two readings for tourmaline (1.624 - 1.644) is noted an inexperienced tester could be: confused, but rotation of the stone on the refractometer's hemisphere shows that the other reading is near 1.62, rather than near the 1.69 for Peridot. Zircon is distinguished easily by its high luster and the fact that its R.I. is usually over the refractometer's scale.

There are two other very rare gemstones that are confused with Peridot- Diopside resembles Peridot closely in both appearance and properties. Depending on the amount of iron present, diopside varies from light to dark green and its indices range from 1.67-1.70 to 1.70-1.725. The birefringence is never over 0.030 which is distinctly lower than Peridot. The optic sign is definitely positive, whereas the middle index of Peridot is almost exactly at the midpoint between the extremes. Diopside almost always barely floats in metbylene iodide; Peridot, on the other hand usually sinks slowly.

The second gem long confused with greenish-brown or brownish green Peridot is the very recently identified Sinhalite. This mineral has minimum and maximum indices that approximate those of Peridot, and the specific gravity is about what iron-rich Peridot shows (near 3.48). The distinguishing feature is the fact that Sinhalite is strongly negative is optic sign i.e. the intermediate index is very close to the high reading.

Valuation and Buying Tips for Peridot

Badly flawed Peridot are worthless for jewelry purposes. Even stones that are slightly flawed to the unaided eye are undesirable and of very little value. Almost very large, fine quality Peridot are not common, their per carat price is never greater than that asked for 20 carat sizes. In fact, extremely large stones, which are difficult to mount and impractical to wear, often cost less than smaller stones of equal quality.

Purity of the green color is also a determining factor. It is be noted that the purer the green color (i.e., the least amount of other color), the more valuable the stone. Another factor to consider is the appearance of the fact edges, a stone that has been handled for some time usually shows evidence of wear (abrasions) along the facet edges. In order to put into top sale able condition, re-polishing is necessary, and the cost of re-polishing should be detected from the price of the stone.



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