Peridot is the gem form of olivine but what is really odd is olivine is not a mineral. If you read Fleischer’s Glossary of Mineral Species, which many use, olivine is not listed. This is because the name’s general use is given to a solid solution series of two minerals, fayalite and forsterite.
Pure forsterite is magnesium silicate, and fayalite is an iron silicate, but they rarely occur in pure form. This state is because magnesium and iron are present in varying amounts in both species. Whichever of these two elements dominates in a mineral determines if it is forsterite with magnesium exceeding iron or fayalite, the iron-rich species.
Olivine is the name assigned to this series, and if the mineral is gemmy, its identity is peridot. In the early days, the gem was known as topazos as it looks like yellow topaz. Another name for peridot is chrysolite, which means gold stone. No matter which name is applied, the stone’s color is always some green shade, which is influenced by traces of chromium or nickel. Peridot is one of the few gems found in only one color. True peridot can range from bright yellow-green and low in iron to brownish-green and high in iron, so olivine is always green.
Historically, the finest peridot gems were mined by the Egyptians on an Island in the Red Sea. In those early days, the green gems were easily mistaken for emeralds. The Egyptians considered it the Gem of the Sun, and it is now the National Gem of Egypt. The Egyptians mined peridot at least 3,500 years ago on El Zabaragad Island, which in modern times is called St. John’s Island. The mining area was referred to as Peridot Hill.
Peridot is a remarkable gem. It forms in the orthorhombic crystal system with a hardness between 6.5 and 7 and has poor cleavage, which means it can be faceted. It has a vitreous luster, and because of its rarity, fine color, and relatively small size as a gem, peridot is considered a precious gem.
If we could reach the earth’s upper mantle beneath our granitic crust, we would find huge quantities of olivine rock. Scientists, using earthquake vibrations and artificially induced earthquakes of the earth, show the upper mantle is made up mainly of olivine, likely mostly peridot. This prompts scientists to estimate that olivine is the most common rock on the earth. Just imagine the size of some gemmy peridot masses in the earth’s mantle. The mineral lends its name to an important mantle rock., peridotite, the rock where diamonds are found. These gems in peridotite originate in the lower mantle and are brought to the crust by volcanic action.
Above ground, olivine forms in many places on earth in limited amounts. It may surprise you to know peridot is also found in space and its gemmy. The earth sometimes is an audience to a comet that zooms into our Solar System, curves around the sun, and heads back out into space. The solar wind causes these comets to develop long sweeping tails that grow as the comet nears the sun. Scientific studies of these comets reveal the tails contain traces of peridot. Stony meteorites are another space rock with ties to peridot. Stony meteorites come in two general forms, carbonaceous chondrites and pallasites. Chondrites look like ordinary earthly stones and are eagerly sought by astronomers as they represent some of the more primitive meteorites to have formed. They contain organic compounds which are very suggestive of life, and as such are studied carefully.
Peridot can be found in many places in British Columbia and is highly sought after. Some areas include Big Timothy Mountain near Hendrix Lake, Prince George area, Dease Lake, Lumby area, Lightning Peak near Cherryville, and Timothy Mountain near Lac La Hache.