The fascinating history of peridot
peridot pronunciation
The olivine mineral group consists of two end-members, forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Peridot, a member of the orthorhombic system, is characterized by thick crystals with wedge-shaped terminations, an oily lustre, and a hardness of 7. It has an imperfect cleavage and a specific gravity of 3.34, with a birefringence of 0.036. Its pleochroic colours are weak, with one ray inclining more to yellow than the others. An iron content of 12-15% gives a good, bright green colour, and traces of chromium or nickel may enhance it. The absorption spectrum of peridot shows three distinct bands in the blue area due to iron, with a narrow core at 497 nm. Anderson & Payne also report vague, weak bands in the orange at 635 nm and in the green at 529 nm. With the help of a 10 lens, these features can easily be detected. Additionally, the iron content inhibits luminescence, and the occasional cat's-eye is found. By using a spectroscope, users can observe all of these features.
directions to ensure a correct absorption picture is given.
Inclusions
Peridot is renowned for its wide variety of inclusions, each of which is unique to certain areas. For example, rectangular 'biotite' crystals are a hallmark of stones from Mount Kyaukpon, Myanmar, while blebs of natural glass and grains of chromite are found in Hawaiian stones. Chromite is also present in peridot from other locations, such as Arizona and the Island of St John (Zeberget). Additionally, characteristic discs known as lily-pads, which often contain a chromite crystal at the center, are a result of decrepitation haloes. Curly hair-like ludwigite and near-black ludwigite rods are a defining feature of peridot from Pakistan. Chromian spinel is a common inclusion in peridot from San Carlos, Arizona, while fluid inclusions are found in some stones but do not indicate a particular deposit. Hercynite is reported in peridot from Kilbourne Hole, Arizona, and chalcopyrite has been found in a peridot from Arizona.
Peridot crystal with Ludwigite inclusions from Suppat, Naran-Kagan Valley, Pakistan.
Occurrences
The Zeberged occurrence (also known as Zeberged, and variously spelt) is located approximately 100 kilometers southeast of the Râs Banâs peninsula in the Red Sea. Here, peridot crystals can be found in outcropping peridotites, with the larger crystals occurring on the eastern slopes of Peridot Hill. Arizona is renowned for its high-quality peridot, much of which is sourced from the area of Peridot Mesa on the San Carlos Apache Reservation in Gila County, where the crystals are found in vesicular basalt. This location is documented in the US Geological Survey Bulletin 1027-N, published in 1956.
In addition, bright green, well-formed crystals can be found in Myanmar, on the northern slope of Kyaukpon in a weathered serpentine, where the crystals are loose. Volume 2 part 2 of Hume’s Geology of Egypt (1935) provides further details of this occurrence, drawing from W. F Moon’s Preliminary Geological Report on Saint John’s Island (Red Sea), 1923 [Sinkankas #4563], which was the first mineralogical and geological study of the area and showed mining in operation at the peridotite cavities containing crystals. The report was published by the Geological Survey of Egypt.
Hawaiian peridot can be found on beaches, while stones from Ameklovdalen, Sondmore, Norway, are Fe-poor and a bright yellowish green. Gem-quality peridot is also found in China in the Zhangjikou-Xuanhua area of Hebei province, as well as in Antarctica and alluvial deposits near the Usambara Mountains in the Umba district of Tanzania.
Cutting of Peridot
Peridot is best cut in the trap-cut (step) style, although oval, round, and pendeloque-shaped mixed-cuts are also popular. In some cases, the large table facet is not cut flat, but with a slight doming. It is important to note that peridot may lose its polish when exposed to hydrochloric or sulphuric acids. In the past decade, high-quality peridot from Pakistan has become increasingly available, featuring well-formed crystals.
Simulation
The characteristic oily luster and strong double refraction of peridot serve to distinguish it from its counterfeits, although there are convincing imitations made in glass and in the composite stone made with a garnet top on a suitably colored glass. A peridot-colored synthetic corundum has been produced, as well as a peridot imitation based on the soudé emerald type of composite stone, the top and base being synthetic colorless spinel and the join across the girdle containing a suitable green coloring matter. In the Summer 2004 issue of Gems & Gemology, a large, apparent peridot measuring 38 mm 23.95 mm 16.30 mm was found to be a glass whose refractive index was over the limits set by the standard refractometer and contact liquid. The usual refractive index for peridot falls between 1.65 and 1.69. EDXRF spectroscopy showed that the material consisted mainly of zirconium with some titanium and silicon, with no correspondence to any known mineral. IR spectroscopy showed two broad peaks associated with glass. X-ray diffraction analysis revealed that the material had no crystal structure and was therefore amorphous.
Keywords:
Peridot, gemstone, physical properties, optical properties, composition, color, uses
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