Lets Talk Gemstones

By Edna B. Anthony, Gemologist

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Peridot, often called olivine and chrysolite, is a gem variety occurring in the solid solution series between fosterite (Mg2SiO4) and fayalite (Fe2SiO4). The members of this most common solid solution series of the olivine group are the primary crystallization products of silica-poor but magnesium and iron-rich magmas. Olivine frequently coexists with plagioclase and pyroxenes in igneous rocks. Magnetite, corundum, chromite and serpentine are its associates in crystalline dolomitic limestone formations. It readily alters to serpentine minerals, such as antigorite, and to talc, limonite and hematite.

In 1772, the German natural scientist Peter Pallas discovered a meteorite that fell to earth about 1749. This meteorite was thought to be a “messenger from heaven” by the inhabitants of the Yenisei region of Siberia. It contained numerous grains of chrysolite. Most of these grains were covered by tiny crystal faces. Others were rounded and lacked crystal edges. Some were large enough to facet.   Iron meteorites from other regions of the earth also harbor these alien crystals that are called “pallasites”.

The Latin word “oliva” gave us the name “olivine” that applies to the group and to the solid solution series. The Greek words meaning gold and stone are the origin of the name chrysolite. Throughout history, this name has been used in conjunction with such terms as “oriental”, “Saxony”, and “Ceylon” to denote numerous yellow and yellow-green transparent gemstones that include topaz, prehnite, apatite, sapphire, chrysoberyl, beryl, tourmaline, and andradite garnet. To avoid confusion, it is recommended that its use with reference to any gemstone should be discontinued. In The Color Encyclopedia of Gemstones, Dr. Joel Arem tells us that “peridot” is derived from the thirteenth-century English “peridota”. Others claim the origin is French. The ancient Romans called the gem “topazus”. This resulted in confusion by historians with the mineral topaz that lingers even today. Further confusion is caused by the use of the term “peridot of Ceylon” to denote honey colored tourmaline found in Sri Lanka.

Forsterite was named to honor the German mineralogist J. R. Forster. Forsterite seldom exists in pure form. The rare, almost colorless to pale yellow or light green mineral is the magnesium-rich end member of the series. Heat and pressure from igneous intrusions into magnesian limestones precipitate the formation of forsterite crystals. Forsterite’s lack of gem quality characteristics preclude its use as a gemstone except as a collector’s item.

Principal sources of forsterite crystals are the Nikolai-Maximilian mine near Zlatoust in the Ural mountains, the United States in Bolton, Massachusetts, and the Vesuvian lava deposits in Italy. Deposits of a banded structure of partially altered serpentine, calcite and forsterite crystals called eozoon are located in Grenville, Canada and near Raspenava in northern Bohemia. The alteration of forsterite crystals to serpentine in cracks resembling branches in the structures were once presumed to be fossilized remains of a large one-celled animal of the early Precambrian period until research revealed the mineral character of the deposits.
The iron content of hortonolite exceeds that of peridot, and manganese appears in its chemical composition. Though usually brown, its colors range from yellow-green to black. Several deposits are located in South Africa, but the O'Niel mine in New York in the United States produces the majority of this member of the series.

Fayalite is the iron-rich end member of the series and was first discovered on the shores of Fayal island in the Azores. It was thought that volcanic rocks there were the source of the crystals. Synthetic fayalite can develop as detritus from hot precipitates from large furnaces. The discarded cinders from such furnaces may have been the source of a number of the crystals. Other deposits are located in Yellowstone National Park in the United States and in the Mourne Mountains of northern Ireland. Fayalite exhibits a hardness of 6.5, a specific gravity of 4.39 and refractive indices of 1.827 to 1.879, each the highest of this solid solution series. Its darker colors of olive green, yellow and brown are usually muddied by its high content of iron. Weathered material becomes reddish to brown with a metallic luster.

The volcanic island in the Red Sea, variously referred to as Topazos, is the Isle of St. John. The Isle of St. John, Zarbargad, and Zebirget were the first known sources of peridot. The Isle of St. John is frequently obscured by fog, and ancient mariners incurred great difficulties to locate it. Its deposits were mined extensively until it was forgotten in the middle-ages. Since its rediscovery in about 1900, it has produced extremely fine material of considerable size. The rich medium green crystals are embedded in veins of nickel ore in peridotite formations.

Kozakov in Bohemia became the source of the gem for the Europeans during the period Topazos, or the Isle of St. John, was “lost”. Chihuahua, Mexico is the site of one of the largest known deposits of peridot, but most of the largest and finest deep green peridot crystals come from deposits near Mogok in Burma. In the continental United States, peridot is found in California, New Mexico, and Arizona. The Navajo Indian Reservation deposits in eastern Arizona yield beautiful material from which cut gems over 5 carats are rare. Tourists sometimes pick up small fragments of peridot from the beaches of Hawaii, and volcanic bombs there often contain crystals. Kenya and the Umba district in Tanzania are the sources in Africa. Peridot deposits of lesser importance are located in New Caledonia, Australia, Ross Island in Antarctica, Finland, Greenland, Italy, Germany, and Minas Gerais, Brazil.

Today, some very fine material comes from Pakistan and China. The recent find of peridot from Pakistan was discovered at altitudes that approach 14,000 feet in the Himalayas of central northern Pakistan near Islamabad, which has produced 10 carat-plus sizes. Norway is the source of lovely pale green crystals that contain less iron and are closer in chemical composition to fosterite than darker peridot. However, the almost colorless gem-quality crystals found at Ratnapura in Sri Lanka possess the chemical composition closest to that of fosterite. It is associated there with olive green material. A trace of chrome and an iron content of 12 to 15 percent produces the most desired deep rich green color of the this gem. Because of properties so similar to peridot, sinhalite was thought to be a brown variety of peridot until the mineral was correctly identified in 1952.

Fine peridot has a velvety appearance quite different from that of emerald and other green gemstones. Asterism and chatoyancy occur but are extremely rare in this olivine material. Such a gem would be highly prized by a collector. With hardness less than quartz, peridot scratches and chips easily. Although it has imperfect cleavage, sharp blows can cause it to fracture. The gem is best suited for use in earrings, necklaces and pendants.

The optical and physical properties of the olivines depend on the composition of the crystals and wide variations exist. The characteristics listed below best represent the averages of peridots used in the gem trade.       
{Editor’s comment: Faceters should be aware that peridot is strongly birefringent (doubly refractive) and can show doubling of the facets. Orient the rough for dopping by looking for the C axis.}

COMPOSITION:                 Mg2SiO4Fe2SiO4            +Mn +Cr        magnesium iron silicate           
CLASS:                               Silicate
GROUP:                             Olivine
SPECIES:                            Forsterite
VARIETY:                Peridot
CRYSTAL SYSTEM:                Orthorhombic
HABIT:                Thick tabular – Short prisms - faces rarely striated
CLEAVAGE:                Imperfect
STREAK:                White
FRACTURE:                Conchoidal
FRACTURE LUSTRE:                 Oily
LUSTRE:                Vitreous to Oily
DIAPHANEITY:                Translucent to Transparent
COLORS:                Yellow-green   bright green   olive green   brownish-green
PHENOMENA:                Asterism and chatoyancy extremely rare
SPECIFIC GRAVITY:                 3.27 to 3.37
HARDNESS:                6.5 to 7.0
TOUGHNESS:                Fair    Brittle
REFRACTIVE INDICES:                1.654 – 1.690                            
BIREFRINGENCE:                +0.036
OPTIC CHARACTER:                Varies          Forsterite - Biaxial positive   Others - negative                               
DISPERSION:                0.020
PLEOCHROISM:                Weak - colorless to pale green, green, olive green
LUMINESCENCE:                None
ABSORPTION SPECTRUM:                Distinct bands at 496, 474, and 453 nm                                       
CHELSEA FILTER:                No reaction
AQUA FILTER:                No reaction
SOLUBILITY:                Slow in HCL to form gelatinous silica
THERMAL TRAITS:                Infusible        Avoid thermal shock
TREATMENTS:                None    
INCLUSIONS:                Biotite; mica crystals; tiny spheres of volcanic glass in Hawaiian peridot that could be mistaken for gas bubbles which would indicate a synthetic gem; smoke veils in San Carlos material from Arizona, - Lily-pad (lotus-leaf) –E. Gubelin surmised that a tiny chromite crystal precipitated from a drop of the chromite-rich mother liquor onto the face of the growing host crystal and the residual liquid spread and was enclosed by the host crystal to create this fascinating and diagnostic disc-like inclusion.