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Peridot: The Gem • Vol. 19: #42 • (10-15-2022) Tidbits of Coachella Valley
Feel adventurous? Here's the recipe for the yellowish-green gemstone called peridot: Melt a load of silicon until it’s runny. Beat in oxygen at the ratio of four parts oxygen to one part silicon. Fold in a generous serving of iron, and add a touch of magnesium to taste. Let cool. Wear with pride.
• Some pronounce it “per-ruh-dot” but most mineralogists say “per-ruh-doh.” Whether or not it settles the matter, there’s a rhyme that says: “A mineral called peridot / is really olivine on show.” The name may have come from the Arabic “faridat” meaning “gem.”
• Eight main elements make up Earth’s crust. The top two are oxygen and silicon. When oxygen and silicon join under heat and pressure, they form silica. There are many types of silicate minerals. The most well-known silicate is quartz, composed of pure oxygen and silicon. Quartz is the second most common mineral in the crust of the earth, after feldspar, which is also a silicate. Silicate materials make up 90% of Earth’s crust.
• Another type of silicate is called olivine (rhymes with green). Olivine is formed when oxygen and silicon join in hot magma underground, along with iron and magnesium. It’s the iron that gives olivine its green color.
• The ratio of iron to magnesium determines the shade of green, ranging from brownish green to pale yellow-green. The more iron there is, the darker the green. The more magnesium, the lighter the green. Other gemstones including rubies, sapphires, and even diamonds, can come in many colors, but olivine is always green. Olivine is the eighth most common mineral in the earth’s crust.
• When magma cools and hardens into rock, olivine is mixed with other igneous rock, like mint-green chocolate chips embedded in a big basalt cookie. Most olivine is cloudy with many flecks and flaws called inclusions. However, there are pockets of pure olivine that are clear as glass, and this gem-quality olivine is called peridot. Peridot shines like emerald when properly cut and polished.
• Peridot is one of only two gems that form in the molten rock of Earth’s upper mantle; the other is diamond. Other gems, including quartz, form in the crust.
• Peridot is the most common gem of the various gems found in meteorites, indicating that other celestial bodies are also rich in silicon, oxygen, iron, and magnesium. Other gemlike minerals found in meteorites include diamond, black diamond (carbonados), opal (found only in Mars-based meteorites), and moldavite (a type of ‘space glass’).
• One of NASA’s telescopes identified olivine in the gassy cloud of a forming star, with crystallized particles swirling in the currents like green rain.
• Olivine/peridot has been found on the surface of Mars by the Mars Rover.
• Some of Hawaii’s lava flows are rich in olivine and peridot. As the mineral erodes out of basalt over millenia, it breaks down into sand. There are beaches featuring green sand, where you can walk on millions of granules of olivine and peridot.
• Cleopatra famously owned an extensive emerald collection, though some historians and geologists theorize that many of her green gems might actually have been peridot.
• The world’s largest cut and polished peridot came from an island off Egypt. It’s almost 312 carats and is on display in the Smithsonian. Keep in mind that an average diamond engagement ring is one or two carats in size.
• Peridot is the traditional gemstone gift for 16th wedding anniversaries and is also the birthstone for the month of August.
