The minerals forsterite and fayalite in the olivine mineral group (there is no individual mineral olivine) form the end-members to the series Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite). An iron content of 12–15% gives a good, bright green and traces of chromium or nickel may enhance it. Peridot is a member of the orthorhombic system in which it forms typically thick crystals with wedge-shaped terminations; twinning is common and some faces are striated. Hardness is 7 and there is an imperfect cleavage; the SG for peridot is usually near to 3.34 and the RI
for the alpha, beta and gamma rays is 1.654, 1.671 and 1.689 respectively, biaxial positive with birefringence usually 0.036. This can easily be detected by the 10 X lens.
Peridot hosts a number of inclusions, some of them characteristic for particular areas. Rectangular ‘biotite’ [name no longer used – refer to dark monoclinic trioctahedral Li-free micas] crystals characterize stones from Mount Kyaukpon, Myanmar. Blebs of natural glass have been found in Hawaiian stones together with grains of chromite; these latter, however, occur in peridot from other places, including Arizona and the Island of St John (Zeberget). The characteristic discs known as lily-pads are decrepitation haloes and often contain a chromite crystal at the centre. Curly hair-like ludwigite and near-black ludwigite rods may be characteristic of peridot from Pakistan.
Chromian spinel seems to characterize peridot from San Carlos, Arizona.Fluid inclusions are found in some stones but do not indicate particular deposits. Hercynite is reported from peridot found at Kilbourne Hole, Arizona. Chalcopyrite has been found in a peridot from Arizona. Curly hair-like ludwigite, and near-black ludwigite rods are characteristic of peridot from Pakistan.
The Zeberged occurrence (the name is a translation and can be variously spelt) is about 100 km south-east of the Râs Banâs peninsula in the Red Sea. Peridot crystals are found in outcropping peridotites with the larger ones occurring on the eastern slopes of Peridot Hill. Much of the finest Arizona peridot comes from the area of Peridot Mesa on the San Carlos Apache Reservation in Gila County, where the crystals occur in vesicular basalt. The location is described in Mineral resources of the San Carlos Indian Reservation, Arizona, US Geological Survey Bulletin 1027-N, 1956.
Bright green well-formed crystals are found north of Mogok, Myanmar, on the northern slope of Kyaukpon in a weathered serpentine in which the crystals are loose. Volume 2 part 2 of Hume’s Geology of Egypt (1935)
gives some details of the occurrence but takes details from W. F Moon’s Preliminary Geological Report on Saint John’s Island (Red Sea), 1923. This is the first mineralogical and geological study of this area and shows mining in operation at the peridotite cavities containing crystals. The report is published by the Geological Survey of Egypt.
Hawaiian peridot is found on beaches. Stones from Ameklovdalen, Sondmore, Norway, are Fe-poor and a bright yellowish green. Gemquality peridot is found in China in the Zhangjikou-Xuanhua area of Hebei province. It is reported from Antarctica and from alluvial deposits near the Usambara Mountains in the Umba district of Tanzania.
The characteristic oily lustre and strong DR of peridot serve to identify the stone from its counterfeits, although there are convincing imitations made in glass and in the composite stone made with a garnet top on a suitably coloured glass. A peridot-coloured synthetic corundum has been produced and also a peridot imitation based on the soudé emerald type of composite stone, the top and base being synthetic colourless spinel and the join across the girdle containing a suitable green colouring matter.
In the Summer 2004 issue of Gems & Gemology a large, apparent peridot measuring 38 mm X 23.95 mm X 16.30 mm was found to be a glass whose RI was over the limits set by the standard refractometer and contact liquid. The usual RI 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 with any known mineral. IR spectroscopy showed two broad peaks associated with glass. X-ray diffraction analysis showed that the material had no crystal
structure and was therefore amorphous.