Parts of Arizona produce most of the world's peridots.
The peridot is a gemstone from the olivine mineral group. The stones have a distinctive yellow-green color that varies in depth according to the amount of ferrous iron present in its formation. In the past, peridots were mainly produced in areas around Egypt and Burma, but the largest deposits are currently found in Arizona and New Mexico. Peridots are usually found in volcanic rock such as basalt, where they form from olivine.
Crystal Formation
Peridots form from olivine during a crystallization sequence in igneous rocks caused by high temperatures and pressure. Gravity settles the small peridot crystals in dunite, a rock mainly composed of olivine, or in peridotite, a rock that contains pyroxene and sometimes garnet and hematite. Olivine has a high melting point, which is one of the reasons for its presence in volcanic rock and the Earth's mantle. Volcanic eruptions may on occasion throw out peridots in molten basalt. The formation process takes millions of years.
Color
The most important factor determining the peridot color is the presence of ferrous iron and magnesium. A higher ratio of magnesium produces a paler stone, while more iron results in a darker tone. The more desirable greens have an Fe:Mg ratio of 1:8. According to Mineral Miners, too much iron produces yellow-green and brown-green stones that are less desirable for jewelry. Peridots have a hardness grading of about 6.5, making them ideal for earrings and necklaces. However, the stones are less suitable for rings because the stone scratches easily and loses its shine.
Arizona Mine
According to the U.S. Geological Survey, the Peridot Mesa on the San Carlos Apache Indian Reservation in Arizona is one of the most productive sources in the world, and it estimates that 80 to 95 percent of the world's peridots are mined here. Peridots are found at depths of between 3 and 35 meters of the basalt that forms the top and sides of the mesa. Two methods are used to collect the stones. Weathering removes some peridots from the basalt and the stones are collected from the canyons and gullies around the mesa. The other method requires blasting and drilling to remove them peridots from the basalt.
Peridot Sizes
Large carat stones are rare. In the past, the Burmese mines produced stones of between 20 and 40 carats. Burmese peridots no longer appear on the market, although it is unclear whether this is because they are mined out or because of government policies. Stones from the Peridot Mesa are typically about one carat, but stones of two and three carats are not uncommon. Stones of five carats and over are rare. Individuals and families living on the Apache Indian reservation legally control mining and marketing of the San Carlos peridots.
Tags: from olivine, Apache Indian, carat stones, ferrous iron, form from
