Rocks keep better stories than they let on. A jeweler mailed the Hope Diamond to the Smithsonian for $2.44 in postage. A crystal you could lose in your palm is older than any rock on Earth. Fluorite gave fluorescence its name, lightning may make the only natural magnet, and the Rosetta Stone turns out to be neither black nor basalt.
Every tile in a Rocks, Gems and Minerals game carries one of these. Open one to read it now, or leave them for the board to hand you.
Gems
nine stories
1Ruby and sapphire are the same mineral
On the tile: Corundum
Hold a ruby beside a blue sapphire and you are holding one mineral twice. Both are corundum, with the same chemical composition and the same crystal structure. The colors come from impurities. A trace of chromium turns the stone red, and only then does it get the name ruby. Iron and titanium turn it blue. Every other color of gem corundum, pink and yellow and green and colorless alike, is filed under sapphire. The most famous divide in colored stones is a pinch of chemistry.
Most gems turn up in several countries. Tanzanite turns up in one, at Merelani in northern Tanzania, near Mount Kilimanjaro. By the most-told account, a Maasai tribesman came across the blue-violet crystals there in 1967, and no other source has appeared since. The stone is a variety of the mineral zoisite, and its color shifts with the viewing direction, so a cutter decides how blue or how violet the finished gem will look. Tiffany and Company named it for the country and launched it with a campaign in 1968.
An opal has no colored ingredient. Inside the stone sit millions of microscopic spheres of silica, stacked in an orderly grid. White light entering that grid is diffracted and comes back out sorted into spectral colors. The size of the spheres decides which colors you see: about a tenth of a micron across returns violet, about two tenths returns red, and the sizes between fill in the rest. Common opal has the silica but not the orderly stacking, so it shows no play of color.
The Shrine of the Three Holy Kings in Cologne Cathedral carries great green stones of about 200 carats each, long taken for emeralds. They are peridot. For centuries green gems were sorted by color rather than by mineral, and peridot spent much of that time filed under topaz as well. Peridot is gem olivine. Most of it formed deep inside the Earth and rode a volcano to the surface, and a very small share of it arrived inside meteorites.
5A precious gem until Brazil dug up too much of it
On the tile: Amethyst1800s
Amethyst once sat beside ruby, emerald and sapphire, priced as their equal and set in bishops' rings and coronation regalia. Then prospectors opened the basalt flows of Brazil and Uruguay, riddled with cavities, some holding several tons of purple crystals apiece. Brazil's deposits swamped the market in the nineteenth century, and fine amethyst has carried a modest price ever since. The mineral itself did not change. It is quartz with a trace of iron, turned purple by gamma rays from radioactive material in the surrounding rock.
Carvers in China and in the Americas worked a tough green stone for thousands of years and called all of it jade. In 1863 the French chemist Alexis Damour tested the material and found the word covered two different minerals: nephrite, an amphibole rich in magnesium, and jadeite, a pyroxene rich in aluminum. Chinese masters had already sensed it. Long before Damour, they noticed the jade coming from Burma was harder and denser, worked more easily and took a higher polish.
Diamond buyers hunt for clarity. Emerald buyers expect the opposite, and the trade has a fond name for it. Most emeralds carry inclusions you can see without a loupe, a mossy tangle that dealers call the jardin, French for garden. Rather than treat that as damage, the trade understands it and accepts it. Emeralds with no eye-visible inclusions are very rare, and that rarity makes them especially valuable, which turns the usual logic of gem grading inside out.
Alexandrite is a rare color-change variety of the mineral chrysoberyl, and the change is dramatic. In daylight it reads green or bluish green. Under incandescent light the same stone turns red or purplish red. The cause is an absorption band at 580 nanometers, which lets the color flip as the light source changes. Abundant deposits were found in Russia's Ural Mountains in 1830, and the gem was named for the young Alexander II, heir to the throne. Gem aficionados describe it as emerald by day and ruby by night.
Amber sits in gem cases beside stones born of heat and pressure, and it belongs to none of them. It is tree resin, hardened and chemically altered over millions of years. Because it came from something living and has no orderly internal arrangement of atoms, mineralogists do not count it as a mineral. Resin flowed to seal wounds in bark, which is why amber so often holds the insects, spiders and scraps of plant that got stuck in it.
The mineralogists of his day sorted minerals mostly by chemical composition, and not very consistently. In 1812 Friedrich Mohs tried the botanists' approach instead, grouping minerals by a physical character: hardness, which he defined as resistance to being scratched. He picked ten reference minerals, from talc at the soft end to diamond at the hard end, and every one but diamond was common, easy to get and cheap. If one scratches another, it is harder. Geologists have carried that test in their pockets for two centuries.
In 1852 the physicist George Gabriel Stokes noticed that specimens of fluorite glowed blue when he lit them with radiation from beyond the violet end of the spectrum. He needed a word for the effect and borrowed the mineral's name, calling it fluorescence. The word then spread into mineralogy, gemology, biology, optics and commercial lighting. The mineral is less reliable than its namesake: many fluorite specimens do not glow at all, and those that do usually shine blue-violet.
3Squeeze a quartz crystal and it makes electricity
On the tile: Quartz1880
In 1880 two brothers in Paris, Pierre and Jacques Curie, put carefully prepared crystals under mechanical stress and measured electric charge on their surfaces. Their test pieces included quartz, tourmaline, topaz, Rochelle salt and cane sugar. The effect stayed a laboratory curiosity for decades. Then Paul Langevin used quartz plates to detect submarines in 1917, work that led to sonar, and in 1926 station WEAF in New York became the first in the country to hold its broadcast frequency with a quartz crystal. Modern timekeeping grew out of that squeeze.
Clear calcite, the variety called Iceland spar, splits a light beam in two, so a line drawn beneath the crystal appears twice. The story goes that Norse navigators used such a sunstone to find the sun through overcast skies. No whole crystal has yet been recovered from a Viking site, though a calcite fragment has turned up at a Viking settlement. What has been found is a calcite crystal from the wreck of the Alderney, an Elizabethan warship lost near the Channel Islands in 1592, lying near a pair of navigation dividers.
5A ventilation tunnel opened into a room of giants
On the tile: Naica Cave2000
In 2000 miners drilling a side tunnel for ventilation under Naica mountain in Chihuahua, Mexico, broke into a chamber about 300 meters down packed with milky beams of selenite gypsum. The largest run over 37 feet long and more than three feet wide, among the biggest natural crystals ever found. They grew because the water in the chamber settled at around 136 degrees Fahrenheit and stayed there, a temperature that feeds gypsum crystals slowly and steadily for ages.
6Fool's gold is named for fire and often holds gold
On the tile: Pyrite
Pyrite takes its name from the Greek pyr, fire, because a lump struck against steel throws sparks. Sixteenth-century wheel lock guns used that trick, clamping a piece of pyrite against a spinning steel wheel to light the powder; flintlocks later switched to flint. The nickname fool's gold came from its brassy color, metallic luster and heft. The joke is only half on the prospector, because gold and pyrite form in the same rocks, and some pyrite holds enough gold to be worth mining. A pin settles it: gold dents, pyrite chips.
Halite is sodium chloride, the mineral sitting on the dinner table. In southern Poland the rock salt at Wieliczka and Bochnia has been mined since the thirteenth century, the oldest undertaking of its type in Europe. Between them the two mines hold hundreds of kilometers of galleries, and the miners who spent their lives down there carved the salt around them into chapels and statues. Both mines, with the Wieliczka Saltworks Castle, are now a World Heritage site.
Magnetite is everywhere. Lodestone, the kind that is already a magnet, is rare, and for centuries nobody could explain why some lumps of one mineral pulled iron. Two clues point upward. Lodestones are found at or near the surface and not down in mines, and their magnetite has been partly oxidized to a related mineral called maghemite. The leading explanation, argued by the retired NASA scientist Peter Wasilewski, is that a lightning strike does the magnetizing.
Muscovite is the pale mica that splits into thin, flexible, transparent sheets. Large sheets came out of Muscovy, the older Russian name for the province of Moscow, and the panes were known as Muscovy glass. That trade name is thought to have given the mineral its own. For centuries those sheets glazed windows in homes and churches across Eastern Europe and India. Colonists at Jamestown had panes of it, and fragments survive in the site's collection.
When the jeweler Harry Winston gave the Hope Diamond, all 45.52 carats of it, to the Smithsonian, he did not hire an armored car. On November 8, 1958, he sent it from New York by registered first-class mail. The postage came to $2.44, with another $142.85 for a million dollars of insurance. Two days later a letter carrier named James G. Todd collected the parcel at the Washington city post office and delivered it to the museum's Gem Room.
Frederick Wells, a manager at the Premier mine near Pretoria, found the Cullinan on January 25, 1905. The crystal weighed 3,106 carats and measured roughly four inches by two and a half by two. Presented to Edward VII on his sixty-sixth birthday in 1907, it was cut in Amsterdam into nine major gems. The two largest, Cullinan I at 530.2 carats and Cullinan II at 317.4 carats, went into the British crown jewels, one in the royal scepter and one in a crown.
The Koh-i-Noor reached England in 1850 weighing 186 old carats, poorly cut, flat-based and mounted in an armlet. Victorian taste wanted brilliance, so in 1852, largely at Prince Albert's suggestion, it was recut as an oval brilliant. What came out weighs 105.60 carats. Whether that counts as improvement or vandalism is still argued. Since the stone entered British hands only queens and queen consorts have worn it, on the strength of a legend that it brings bad luck to any man who does.
On the night of October 29, 1964, two men scaled a fence at the American Museum of Natural History while a third circled the block in a white Cadillac. They climbed a fire escape, swung on a rope through a fourth-floor window, and took 24 stones, among them the Star of India, a 563.35-carat sapphire. The museum's defenses were largely notional. Batteries in the display-case alarm had been dead for months, and the gem hall's windows were propped open for ventilation.
In 1924 Wesley Oley Basham, a workman at a diamond mine near Murfreesboro, Arkansas, turned up a 40.23-carat crystal, still the largest diamond ever found in the United States. It was dubbed with his own nickname: Uncle Sam. Cut down to a 12.42-carat emerald-cut gem, it is now in the Smithsonian. The ground it came from is the odder part. That 37-acre patch sits atop an ancient volcanic vent, the Prairie Creek Pipe, whose eruption more than 100 million years ago carried diamonds up from the mantle. Today it is a search field inside a state park, open for a small admission fee.
Green diamonds get their color from radiation, usually uranium in rocks near the surface, and the stain sits on the skin of the crystal. Cutting normally takes it off, so a light green rough often comes out of the wheel colorless. The Dresden Green kept its color. At roughly 41 carats it is the largest green diamond known to be naturally colored, and in 1988 two senior gemologists examined it in Dresden, partly to learn how to tell natural stones from irradiated ones.
Generations learned that the Rosetta Stone is black with white lettering. It is neither. The rock is granodiorite, gray with pink in it, and the black was a coating of carnauba wax while the white was paint added in 1981 to make the text legible. Conservators took it all off in 1999 for an exhibition at the British Museum. They deliberately left one small square at the bottom left corner untouched, so visitors can still see the darkened wax.
The Pilgrims made their first North American landfall on Cape Cod, not at Plymouth, and the earliest accounts of the colony mention no rock at all. The boulder became famous in 1741, when Thomas Faunce, born about 1647 and by then in his nineties, spoke up to stop a wharf being built over it and said his father had pointed it out as the landing place. The stone itself is Dedham granite about 600 million years old, carried to the beach by a glacier.
The Stone of Destiny, a block of sandstone used to install Scottish kings at Scone, was taken to Westminster in 1296 and stayed there for seven centuries. On Christmas morning 1950 four Glasgow students removed it from the Abbey. A passing policeman found two of them parked outside in a car, apparently a courting couple, and chatted before sending them on their way, unaware that a broken piece of the stone was hidden inside. It went back to Scotland in 1996, and now travels south only for coronations, as it did for King Charles III in 2023.
The oldest terrestrial material anyone has found is a zircon grain from the Jack Hills of Western Australia, dated at about 4.4 billion years. It is not part of the rock that holds it. The zircons are loose grains that weathered out of far older rock and were swept into sediment, and they survived because zircon is extraordinarily durable. Oxygen isotopes locked inside them suggest their parent magma came from rock that had already met surface water.
2She found the inner core in waves that should not exist
On the tile: Inge Lehmann1936
Earth was understood to have a molten core inside a rocky mantle, and not much more than that was known. Then a large earthquake in New Zealand in 1929 sent a few waves where the model said none could arrive. They should have been deflected by the core, yet seismic stations recorded them. The Danish seismologist Inge Lehmann worked out that they had traveled into the core and bounced off a boundary within it, and her 1936 paper proposed a solid inner core inside the molten outer one.
Marie Tharp turned columns of sounding data into drawings of the sea floor, and a deep valley kept appearing down the middle of the ridge she was mapping. She rechecked her calculations again and again. When she showed Bruce Heezen, he groaned that it could not be, because it looked too much like continental drift, and he brushed off her reading as girl talk. It took him about a year to come around, and what changed his mind was earthquakes lining up along the rift.
In 1788 James Hutton took a boat along the Berwickshire coast and landed at Siccar Point, where beds of one rock stand on end and beds of another lie gently across their sawn-off tops. Hutton read that junction as a full cycle: sediment laid down, buckled upright, worn flat, then buried again, which demanded far more time than his contemporaries allowed. John Playfair, who was there, later recorded that "The mind seemed to grow giddy by looking so far into the abyss of time."
5The deepest hole ever drilled stopped at 7.6 miles
On the tile: Kola Borehole1989
Soviet engineers on the Kola Peninsula spent about twenty years drilling straight down to see what the crust is made of, aiming for nine miles. By 1989 they had reached 12,262 meters, roughly 7.6 miles, and it is still the deepest hole anyone has dug. Then the heat beat them. The 2.7-billion-year-old rock down there sat at about 180 degrees Celsius, nearly twice what they had predicted, hot enough to deform bits and pipe and soften the rock itself.
Whether the great bowl in northern Arizona was volcanic or the scar of an impact was argued for decades, and the two leading disputants both died without an answer. Eugene Shoemaker sent shocked sandstone from the crater to Edward Chao, a mineralogist at the U.S. Geological Survey, who recognized coesite, a form of silica that takes enormous pressure to make. Chao, Shoemaker and Beth Madsen published the finding in Science in July 1960, and the debate was over.
7A shark's head solved the riddle of tongue stones
On the tile: Nicolas Steno1667
In October 1666 fishermen off Livorno landed an enormous shark, and the grand duke had its head sent to Nicolas Steno to dissect. Steno looked at the teeth and recognized them at once. They matched the stony objects called glossopetrae, tongue stones, that turned up inside rocks far from any sea. He argued the tongue stones were shark teeth, and then worked out how solid objects come to sit inside solid rock. Stratigraphy grew from that question.
Walk down into the Grand Canyon and at one contact you cross about 1.2 billion years of Earth history in a single stride, roughly 30 percent of the planet's past. Below the line lie the Vishnu basement rocks, around 1.75 billion years old. Directly above them sit layered rocks of about 510 million years. The surface between records mountains raised by tectonic forces, worn down to base level, then buried under the sediments of advancing seas.
At Gubbio in Italy, Walter Alvarez found the Cretaceous boundary exposed as a clay layer about half an inch thick. Analysis turned up nothing until his father Luis suggested looking for platinum-family metals, which are scarce in Earth's crust and plentiful in asteroids. The radiochemists Frank Asaro and Helen Michel joined, and found far more iridium in that clay than in the rock above or below it. The four published their impact theory in 1980.