The power of gems
There, a row upon row of white plush stands, illuminated by hidden reflectors, blaze with multicolored droplets of fire. How joyful, how enchanting is their magical glitter to our eyes in the darkness of night!
In the middle, there are two brilliants, the size of a hazelnut each. Neither their facets nor their contours are discernible. These are not stones – these are two strange mysterious lights. They blaze, shimmer, flash, glitter and laugh with thousands of elusive, sparkling and roguish smiles. They lure, and promise, and deceive…
A.I. Kuprin, "Brilliants"
Emerald, the sacred stone of Incas and many other native American cultures, was mined in the Eastern Cordillera for at least a thousand years before the Spanish conquest. As soon as the Spaniards arrived on the Caribbean coast of what is now Colombia, they knew they were on to something as big as, and certainly much more real than, the elusive El Dorado. Generous welcome gifts and ransoms paid in emeralds only whetted the appetite of conquistadors. By 1540s, thousands of emerald crystals had been looted and the legendary Somondoco mine seized from the Muisca Indians. The Muzos, who lived farther west, did not give up fighting for another two decades and many of them chose death over divulging the secret of emerald mines. By the early 1600s, the entire region came under Spanish control, and the enslaved Indians were forced to dig emeralds for the court in Madrid.
When Botswana became independent in 1966, its colonial “legacy” included 3.5 miles of tarred roads, 3 secondary schools and a gross domestic product of $50 million US. Today, it has 3.5 thousand miles of roads, more than 300 secondary schools and a GDP well over $5 billion US. It is one of the most prosperous countries in Africa, and a popular destination for foreign investments and ecotourists. To a great extent, Botswana owes its success to the fabulously rich diamond deposits discovered by De Beers geologists under the Kalahari sands in the 1960s and early 1970s. The Botswanan mines produced 32 million carats (~14,100 pounds) of diamonds in 2005 – more diamonds by value ($3 billion US) than any other country in the world.
One could recall many more examples where gemstones seem to have had the mystical power to alter the course of history. Whether you believe that they do, or think gems are just what we make of them, rest assured: Gem mining, fashioning and trade have been and still are important elements of the social, political and economic landscape in many cultures. What exactly makes diamonds, emeralds and other gemstones so special and unlike anything else?...
Gemstones: What's in the name?
In such moments, when her eyes sparkled, her cheeks reddened, and her whole frame became animated, it was pretended that the opal clasp amid her tresses, the ornament which she never laid aside, shot forth the little spark, or tongue of flame, which it always displayed, with an increased vivacity. In the same manner, if in the half-darkened hall the conversation of Hermione became unusually animated, it was believed that the jewel became brilliant, and even displayed a twinkling and flashing gleam which seemed to be emitted by the gem itself, and not produced in the usual manner, by the reflection of some external light.
Sir Walter Scott, "Anne of Geierstein"
If you are looking for a “scientific” definition, gemstones are natural and man-made materials that are sufficiently attractive, durable and rare to be used in jewelry, ceremonial items, or as tangible assets. The majority of gemstones found in nature are minerals, but some are rocks (i.e. assemblages of minerals), while others are organically produced materials. For example, lapis lazuli comprises crystals of blue lazurite, calcite, pyrite and several other minerals, and thus should be classified as a rock. Jasper, one of the most popular materials with stone carvers, is a metamorphic rock formed from silica-rich sedimentary and volcanic rocks under high temperatures and pressures. Pearls consist of tiny crystals of Ca carbonate (usually aragonite) interspersed with a protein-rich, fingernail-like organic binder, termed conchiolin. To add to this complexity, water makes up about 2 to 4% of every pearl. Finally, amber is a glassy material made up of cross-linked and polymerized organic molecules, called terpenes. Hence, neither amber nor pearl are minerals – although they are certainly gemstones!
Gemstones that are minerals comprise intricate 3-D arrangements of atoms held together by chemical bonds. Such arrangements, called crystal structures, as well as the assortment of atoms building the structure, are the ultimate characteristics of any mineral. Both crystal structure and chemical composition can be quite simple (as in diamond), or immensely complex (as in tourmaline).
Many gemstones bear the same name as the mineral that they represent (diamond, topaz, malachite). Others may derive their name from a mineral variety notable for its color or some physical property (amethyst = purple quartz), or have a name of their own (padparadscha, for example). In the latter cases, the name must have deep roots in tradition and history to have persisted to the space age.
Man-made gemstones that replicate natural ones (i.e. have the same composition and structure) are referred to as synthetics (synthetic ruby, synthetic emerald, etc.), whereas materials that do not have a natural counterpart are called simulants. Some simulants bear commercial names that reflect their real chemical composition (e.g., CZ for cubic zirconia), others are simply “bell-ringers” bound to catch your ear (fabulite, tanzanion, etc.).
Gem Deposits 101: Where do gems come from?
Natural gemstones form in a variety of ways. Some, like diamond, Mg-Al garnet (pyrope) and peridot, crystallize under tremendous pressures and temperatures in the Earth’s mantle. They are then transported to the upper crust by blobs of molten rock (magma) that solidify at depth or erupt explosively. Instead of volcanoes, these eruptions produce carrot-shaped structures (“pipes”) dipping from the Earth’s surface into the crust. That is how the diamond-bearing kimberlite pipes, discovered recently in Canada’s North, formed tens of millions of years ago. On their way up, these “mantle gems” have to endure a lot of abuse because they are traveling in hot magma charged with H2O, CO2 and other highly reactive chemical components. In fact, many diamonds probably do not survive this trip because they react with oxygen and dissipate into CO2. Some gemstones crystallize from magma, and are then brought to the surface by bursts of basaltic lava. Examples are Australian sapphire and pyrope garnet from the Kingdom of Fife locally known as the “Elie ruby”.
Among all igneous rocks, miarolitic pegmatites are by far the most important source of gemstones. These unusual coarse-grained rocks originate from felsic melts enriched in such rare elements as lithium, beryllium, boron and cesium. Their exact physical nature and provenance are still debated. When such melts solidify relatively close to the surface, they fractionate a fluid that may become trapped within the pegmatite and cool into hollow pockets (miaroles) lined with crystals of feldspars, quartz, tourmalines, beryl, topaz and other gemstones. Inside such pockets, crystals can grow to gigantic proportions. For example, topaz and tourmaline crystals well in excess of one meter in length have been unearthed from Brazilian pegmatites. Unfortunately, very few of such rarities ever make it to the museum shelf, most are “minced” into gems that command a much higher profit.
Gemstones also occur in metamorphic environments. In some cases, their crystallization requires intensive exchange of heat and chemical elements between a hot intrusion of igneous (usually felsic) rock and cooler country rocks. In this manner, emerald and alexandrite formed at the contact between pegmatites and serpentinites in the Urals (Russia), sapphire at the contact of pegmatites with marble in Kashmir, and red spinel in skarns high in the Pamir Mountains. The formation of other metamorphic gemstones (like the purplish red garnet almandine, blue tanzanite and even some rubies) does not require an intrusion – a few million years under high temperature and pressure will do the trick. It is hard to believe that the famous Burmese rubies from the Mogok valley were nothing but layers of clay in limestone “just” 150 million years ago.
Gemstones in igneous and metamorphic rocks commonly form well-shaped crystals. Their symmetry (number and mutual orientation of crystal faces) depends on the structure of a mineral. Note, however, that the majority of gems used in jewelry have been shaped by man, not nature. Even the so-called crystal points, sold for healing and meditation, have usually been refaceted and polished (I am guessing, to enhance their metaphysical powers).
Nothing is eternal in this world, and neither are the igneous and metamorphic rocks that host gemstones. Eventually, weathering and erosion will reduce them to piles of rubble on a hillside, or layers of sediment on a stream bed. Many gemstones, thanks to their durability, will survive this crucible of nature and accumulate where stream velocity is retarded, forming gem gravels. At present, gem gravels are the most important source of gemstones because they (i) are easier to mine than any primary source (try to yank enough sapphires out of basalt to make a living!), (ii) can be worked using primitive and inexpensive mining techniques, (iii) have an enhanced gem content, (iv) typically contain a higher-quality material than the primary source, and (v) often contain several types of gemstones derived from different primary sources. For example, almost all of the world’s rubies and sapphires, and some 2% of gem diamonds are mined from placer deposits. The island of Sri Lanka alone produces 40 different types of gems, including ruby, sapphire, padparadscha, chrysoberyl, spinel and garnet.
There are also gemstones that actually form at the surface of the Earth, either as part of the rock cycle, or as products of biological activity. For instance, malachite is a characteristic mineral of the oxidation zone of copper deposits. When exposed to air and moisture, primary copper ores readily oxidize to form hydro-carbonates and other secondary minerals, including malachite. This fascinating mineral forms concentric aggregates of fine radiating crystals (mineralogists call them botryoidal or colloform aggregates) lining cavities and cracks in the brown oxidized ores.
Both amber and pearls are produced organically. Amber is a fossilized resin secreted by the bark of some pine-like and leguminous trees in a (sub)tropical forest. The transition from resin to amber requires time (by different estimates, from 2 to 10 million years) and just the right kind of “entombment”. For instance, all Borneo amber comes from sands, whereas clays of the same age contain copal, i.e. partly fossilized resin rich in liquid components. Pearls are concretions built by shelled mollusks around an invader (like a food particle or small crustacean) lodged between the shell and mantle. An incomplete concretion attached to the shell on the inside is called a blister pearl. Pearls have been around for probably 500 million years, although the oldest known fossil pearl is “just” 225 million years of age. Today, the most valuable pearls come from tropical marine bivalves known as pearl oysters.
Very few gemstones can be used “as mined”, and most would require a serious face-lift before they can dazzle anyone. Even pearls are sometimes “skinned” by careful filing or sanding to remove blemishes on their surface. Natural crystals, no matter how perfect, are covered with etch pits, striation, growth sculpture, tarnish, or “sprinkled” with other crystals. Inside the crystal, one is also likely to find numerous imperfections – fractures, bubbles of fluid, mineral inclusions, etc. It is now up to a skilled lapidary, faceter or brillianteer to unveil the stone’s beauty with as minimal a loss of carats as possible.
Gemstones can be fashioned into a virtually limitless number of forms (the only real limit here is lapidary’s imagination). They can be faceted, shaped into cabochons, tumbled into beads, laid in mosaics and inlays, or carved into sculptures, cameos and intaglios. Typically, a piece of rough is first sawed and trimmed with a metallic blade impregnated with diamond grit. After trimming, it is preformed to the desired shape using grinding wheels and abrasive powders, and then polished to a mirror-like finish.
Art in gems
Mr. Rolles opened the case, and drew a long breath of almost horrified astonishment; for there lay before him, in a cradle of green velvet, a diamond of prodigious magnitude and of the finest water. It was of the bigness of a duck's egg; beautifully shaped, and without a flaw; and as the sun shone upon it, it gave forth a lustre like that of electricity, and seemed to burn in his hand with a thousand internal fires. He knew little of precious stones; but the Rajah's Diamond was a wonder that explained itself; a village child, if he found it, would run screaming for the nearest cottage; and a savage would prostrate himself in adoration before so imposing a fetish.
R.L. Stevenson, "The Rajah’s Diamond"
Have you wondered why gems shine and sparkle the way they do, and some even seem “to burn with a thousand internal fires”? Of course, much of it has to do with their optical properties. Light passing from air into a crystal refracts, i.e. changes its path and slows down. This decrease in light velocity can be quantified as the refraction index (RI) of the crystal. For example, diamond retards light to 124,000 km/s, i.e. by a factor of 2.42 relative to its velocity in vacuum. All common natural gemstones have refractive indices lower than diamond’s 2.42. However, scientists learned to grow gem-quality crystals of synthetic moissanite (SiC) and rutile (TiO2), both of which are significantly more refractive than diamond (RI > 2.60). Only isotropic materials (i.e. highly symmetrical crystals and glasses) have a single RI, most gemstones have two or three different indices.
Closely related to refraction are the phenomena of total internal reflection and dispersion. Total internal reflection occurs when light passing through a crystal strikes the crystal-air boundary at an angle exceeding some critical value. As a result, the light is reflected from that boundary back into the crystal. Refraction and internal reflection work in tandem to give gems their brilliance. Dispersion occurs because light of different wavelengths is refracted differently by the same crystal. Crystals with a high value of dispersion separate white light into spectral colors, much like in Newton’s experiment with a prism. The higher the dispersion, the more effective is the color separation (or, as gemologists say, the stronger the fire). Dispersion can be quantified by measuring the difference between refractive indices for different wavelengths.
After nature has played its part, gemstone’s fate is in the hands of a lapidary or, in the case of diamond, a whole team of cutters. In addition to having the required technical and artistic skills, they must understand the properties of gemstones. This is best exemplified by gem faceting. Before a rough stone is cut, it has to be carefully examined for internal characteristics. Inclusions and fractures not only diminish the value of a faceted gem, but also affect its mechanical properties. Bubbles of fluid can overheat and burst during faceting, damaging the gem; cleavage planes too close to a facet will cause chipping and uneven polish, and so on. Many colored gemstones are zoned or show different colors depending on the angle of viewing. A crystal of blue corundum can yield a sapphire of purplish or greenish hue, depending on whether the upper facet (table) is cut across the crystal or lengthwise. Hence, the distribution of color will determine the orientation of a faceted stone within the rough and, oftentimes, also the type of cut. A cutter also has to make sure that light entering through the upper set of facets (crown) is reflected internally from the lower set (pavilion), with as little light lost through the pavilion as possible. The reflected light will illuminate the gem from within and then escape through the crown, enhancing gem’s brilliance and fire. All these parameters have to be factored in when calculating the number of facets, their orientation, and angles between them.
Indian craftsmen were the first to experiment with faceting. One of the most primitive forms was the table cut popular through the Renaissance. In the late 1400s, the rose cut was developed by Dutch and Flemish faceters. The earliest cut to have a recognizable crown, table and pavilion was the old-mine cut designed for Brazilian diamonds that flooded the European market in the 1700s. This form gradually evolved into the round brilliant cut perfected by Marcel Tolkowsky in the early 1900s.
The round brilliant revolutionized the cutting industry, producing many new forms that have since become very popular, including the trilliant and pear. The ancient table cut also evolved with time, giving rise to a variety of step-faceted designs, like the famous emerald cut. Some classical cuts (Portuegese, for example) are intricate combinations of dozens and, sometimes, over a hundred facets. Recent technological advances in equipment enabled fashioning of very small stones, as well as such innovative approaches and styles as concave facets and geometric patterns carved or drilled into a gem. Most of these innovations are far too expensive for the mass market, and can only be seen in one-of-a-kind designer gems.
Opaque (non-transparent) gemstones, too, will reveal their inner beauty only to the most skilled and imaginative lapidary. Some of these gemstones (like charoite or malachite) sport phantasmagoric color patterns and textures; others, called "phenomenal" gemstones (like spectrolite, for example), exhibit various optical effects: chatoyancy, iridescence, asterism, etc. In either case, a random cut through a piece of rough gemstone will achieve nothing or even ruin the stone. An experienced cutter will know exactly where the blade has to go to expose the best colored, best textured, or most optically spectacular part of the stone, while keeping stone's integrity uncompromised and losses at their minimum.
Gems in art
Holmes unlocked his strong-box and held up the blue carbuncle, which shone like a star, with a cold, brilliant, many-pointed radiance.
A. Conan Doyle, "The Adventure of the Blue Carbuncle"
The beauty and magic of gems have inspired innumerable works of art. “Your mouth’s like of flaming rubies treasure, whilst mine eyes a vessel full of pearl tears” – hard to believe this was written more than a thousand years ago by a poet blind from birth. For many artists, gems held more than just a metaphorical value. For instance, Johann Wolfgang Goethe and John Ruskin were prominent mineral and gemstone collectors of their time. Ruskin is also known for his influence on the Arts & Crafts movement (the one that introduced Celtic and other folk motifs in jewelry), and donating a 167-carat ruby and 133-carat diamond to the British Museum. For the sake of objectivity, we could also recall examples where a crash course in gemology would have certainly been to an artist’s benefit. Sir Arthur Conan Doyle, for one, thought carbuncles could be blue, even though that name was used in the old days strictly for red gems, like ruby or pyrope.
Despite their eventful past, surprisingly few historic gems have made it to the movie screen or pages of adventure novels. The most notable exception in contemporary art is Cameron’s Coeur de la Mer, sketched off the ill-reputed Hope diamond, which at the time of Louis XIV was indeed shaped like a heart and known as le Diamant Bleu de la Couronne. The tales of treachery and opulence, surrounding Hope and the like, have, however, inspired countless gem “biographies” spiced with fictitious events just enough to keep the reader entertained. Of course, the fact that neither the Moonstone, nor the Rajah’s Diamond, nor the Garnet Bracelet ever existed hardly makes reading about them any less captivating, does it?..
Would you like to learn more about gemstones?
University of Manitoba is one of very few universities in North America where gemology is part of the curriculum. Presently, it is offered as an undergraduate elective course at the Department of Geological Sciences. Click here for further information.