PERIDOT: THE GEM
• Here’s the recipe for the 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.
• 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 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 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 8th most common mineral in Earth’s crust.
• When magma solidifies 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 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.
• Some pronounce it “peri-DOT” but most mineralogists say “peri-DOUGH.” There’s a rhyme: “A mineral called peridot / is really olivine on show.” The name may have come from the Arabic “faridat” meaning “gem.”
• 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.
• Some of Hawaii’s lava flows are rich in olivine and peridot. As the mineral erodes out of basalt over the eons, it breaks down into sand. There are beaches featuring green sand, where you can walk on millions of granules of olivine and peridot.
• The San Carlos Apache Reservation in Arizona is the source of nearly 90% of the world’s supply of peridot.
• Olivine/peridot has been found on the surface of Mars by the Mars Rover.
• 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.
SARDONYX: A GEM
• Sardonyx was the original but unofficial birthstone for August dating back centuries. When the American Gem Trade Association and Jewelers of America standardized monthly birthstones in 1912, peridot was added as a second August birthstone. Spinel was added as a third August birthstone in 2016 in an effort to modernize and update the birthstone list.
• June and December are the only other months with three birthstones but various other months have two: the traditional, and the modern.
• Chalcedony [“cal-SAID-any”] is another type of silica, classified as a “microcrystillane form” of silica because the crystals are too small and tightly knit to see without a microscope. Chalcedony is named after an ancient Turkish city called Chalcedon, now a district in Istanbul. The name springs from the Phoenician term meaning “new town.” Chalcedony encompasses an entire family of minerals that have different form and coloration depending on the circumstances of their creation and other minerals present at the time.
• Agate is a translucent variety of chalcedony; jasper is an opaque form. Onyx is chalcedony that’s striped white, grey, and black. Onyx got its name from the Latin word meaning “fingernail” or “claw” because the white bands are the same color as nails. Sard is another type of chalcedony that’s primarily red, orange, or brown because it contains iron oxide. The mineral sard takes its name from an ancient city called Sardis, the capital of the territory of Lydia located in what is now Turkey. The mineral was common in the area.
• When sard and onyx are mixed together, the result is called sardonyx. The stone is an intriguing mix of colors with sard responsible for the red-toned hues and onyx contributing white and black streaks.
• Sardonyx is easily carved and has been used throughout history to create bas-relief cameos in stone, commonly depicting the faces of gods, kings, heroes, and beauties. Ladies sported cameos of the face of Venus to bring love; soldiers wore the face of Mars or Hercules into war for courage and protection. Later, artists carved entire three-dimensional scenes depicting battles, coronations, weddings, funerals, and myths. These works of art are called intaglios, from the Italian word meaning “to engrave.”
• Roman rulers used signet rings and seals made of sardonyx in order to imprint wax insignia on important documents because melted wax doesn’t stick to it.
• Sardonyx has historically been a popular gem because it’s always been inexpensive enough so that even the common people could afford a few pieces, unlike more expensive stones like diamonds, rubies, and sapphires.
• One of the most well-known sardonyx cameos depicts the profile of Queen Elizabeth I. According to historical myth, she gave the item to Robert Devereux, the 2nd Earl of Essex, as a gesture of their friendship. The talisman didn’t work very well, as she had him beheaded in 1601. The cameo now resides in the Royal Collection.
• The city of Oberstein, Germany, is world headquarters for cutting and dying minerals in the chalcedony family in order to enhance their colors. Carbon deepens the blacks; iron brightens the reds. Part of this talent with improving these gemstones is due to large local deposits of these minerals, along with a strong local knowledge of the chemistry of dyes. Additionally, in the 1800s, ships traveling back to Germany from Brazil used agate-rich gravels as ballast in their ships, providing a cheap easy source of materials to practice on.
