Sapphire

One of the official state gemstones of Montana, sapphires hold a special place in the Treasure State where they are readily found. Sapphires and rubies are both corundum, an aluminum oxide mineral typically found in crystalline form. The only difference between the two is the presence of chromium. If the corundum is red, it’s ruby. Otherwise, it’s always a sapphire.

One of the most desired types of gemstones, the most well-known sapphire hue is deep blue, but these gemstones are found in pink, green, violet, orange, purple and even brown. When they’re not blue, they’re referred to as fancy sapphires. These colors are because of the varying degrees of chromium, titanium oxide and iron within the stones. Sapphires also possess a trait called asterism where needle-like inclusions create the appearance of a six or twelve-patterned star. Beyond this unique characteristic appreciated by faceters, sapphires have a Mohs Amy Grisak; Getty Images/Science Photo Library value of nine, just below a diamond, making them extremely durable and an excellent choice for jewelry.

Tiger’s Eye

This distinct gem has a long history of fending off the “evil eye,” with its resemblance to a cat’s eye. In gemology, this trait is called chatoyancy, a French term meaning “shining like a cat’s eye.” When there are crocidolite (blue asbestos) fibers within cabochon-cut gemstones running parallel to each other, the rounded surface allows the light to reflect in a way that gives the tiger’s eye its signature look. Originally, scientists thought this phenomenon occurred when the crocidolite within the stone was changed by iron oxide and replaced with silica. But even though the coloration comes from this process, some researchers believe it’s actually crocidolite inclusions within columns of quartz within the stone that form the distinct paralleling nature.

Regardless of how it formed, tiger’s eye is a favorite gem for tumbling and with a Mohs value of seven, it’s a versatile stone for a multitude of uses. While it’s a ubiquitous stone these days, in the 1870s a single carat of tiger’s eye was worth an ounce of gold.

Unakite

Unakite is a terrific example of when a gemstone is a true rock as this beautiful pink and green specimen is a composite of metamorphic rocks including orthoclase, epidote and milky quartz. It’s formed during hydrothermal metamorphosis when the epidote replaces the silicate minerals, primarily plagioclase, within the granite. The epidote is green within unakite, while the pink orthoclase feldspar and quartz create the colorful speckling.

First found in the Unakas Mountains of Western North Carolina and Eastern Tennessee, it’s sometimes found in the rivers of the region, along with the beaches of Lake Superior where glaciers deposited the metamorphic rocks. With a Mohs rating of six to seven, unakite is among the types of gemstones that tumble well. It has been used to make small sculptures or is cut for jewelry. As eye-catching as it is, unakite is also valuable in construction on many levels, including being used as trim along the front steps of the south entrance of the Smithsonian National Museum of Natural History. It is also sometimes used less visibly as crushed stone in highway construction.

Vesuvianite

Sometimes called idocrase, vesuvianite was originally found along Mount Vesuvius in Italy, which buried the nearby inhabitants of the city of Vesuvius on August 24 in 79 AD, ironically during the festival of Vulcanalia, the god of fire. In the world of gem cutting, vesuvianite often refers to the rough stone, while the faceted gems are called idocrase.

Regardless of the name, this is a calcium-aluminum-silicate mineral that forms in a tetragonal structure. Its most popular colorations range from yellowish green to brownish or olive green, although there is a blue version called cyprine that derives its color from trace amounts of copper. With a Mohs value of six, vesuvianite isn’t a very hard stone and is often used for larger jewelry and sculptures. In its green coloration, it’s sometimes mistaken for other types of gemstones like peridot, although vesuvianite is far rarer.

White Topaz

While topaz is found in practically the entire color range, the white topaz is the clear version and boasts a similar appearance to a diamond. Outside of cost, there are distinct differences between the two types of gemstones. Topazes and diamonds are closely alike in clarity and color, but brilliance is where diamonds shine. Hardness is another determining factor. Diamonds reign supreme rating at Mohs 10, while topazes register as a Mohs eight, considerably less durable with a greater risk of scratching.

Topaz is created when water and magma react during the metamorphic process creating pegmatite featuring natural topaz that is typically initially clear. While the wide variety of colors is because of impurities, such as chromium replacing the aluminum within the stone, white topaz is the gem in its purest form. Specific hues are also created with heat, irradiation or the application of metal oxides to enhance colors. Topaz also exhibits pleochroism where the gem exhibits different colors depending on its angle, although the white topaz tends to remain consistent in its coloration.

Xenotime

On occasion, there are types of gemstones cut from this rare earth mineral, often found in yellowish-orange to reddish-brown hues, although high enough quality stones to facet are rare.

Like topaz, xenotime is found in pegmatite formations, as well as igneous rock and gneiss. Uranium and thorium are often found within this stone, creating natural radioactivity, although it is more commonly seen as a source for the transition metal yttrium, which is used as an alloy in the production of camera lenses and lasers. Its name is derived from the Greek terms for “vain” and “honor” in an early scientist’s snarky rebuke of another. Initially, Swedish chemist Jöns Jacob Berzelius believed he discovered a new element within the xenotime. This turned out to be the already known yttrium, which prompted mineralogist François Sulpice Beudant to throw down a bit of shade on the claim.

Yellow Kunzite

True gemologists might shudder at the inclusion of yellow kunzite in this types of gemstones list, but it’s an example of when marketing can be misguided. As a rule, kunzite is a pink to light purple variety of spodumene, a lithium-rich mineral found, once again, in pegmatite formations. Manganese gives kunzite those attractive colors. When the gem is yellow, it’s typically just called yellow spodumene. The name change might be a matter of one word sounding more appealing than the other, but it is still misleading as kunzite implies a specific hue. With a Mohs value of six and a half to seven, it is not a very durable gemstone, but it’s possible to find specimens of 20 carats or more. Spodumene, in general, is an important source of lithium, which is critical for car batteries, phones and medicine. It is mined in Afghanistan, Pakistan, California, North Carolina and South Dakota.

Zircon

Not to be confused with the synthetic cubic zirconia, zircon earned its place as a popular gemstone 2000 years ago. Found in sand and as part of many of the rocks throughout the world, zircon is one of the oldest minerals on earth. Because of its uranium content, scientists in Australia dated it back 4.4 million years. Not all zircons are radioactive, but those that are can be heat-treated to stabilize the integrity of the stone by slowing the degradation of the crystalline structure. In their natural form, zircons are found in colors ranging from clear to yellow, green, purple, brown and grays, which are typically caused because of radiation or impurities. Blue zircons, which have been popular since Victorian times, are created through heat treatments.

This story about types of gemstones by letter appeared in Rock & Gem magazine. Click here to subscribe. Story by Amy Grisak.

The post Types of Gemstones By Letter (S-Z) first appeared on Rock & Gem Magazine.

“A diamond is forever,” the classic advertising slogan of the De Beers Diamond Syndicate, refers to both the durability of diamonds and to the love relationships they represent. While scientists don’t profess to be experts on matters of the heart, they do know about gemstone durability, especially the Mohs Scale of Hardness. And considering recent scientific revelations, the diamond slogan could be rewritten as “a zircon is forever.”

Zircon the Gemstone

Zircon (zirconium silicate, ZrSiO2) consists of silicon, oxygen and zirconium. With its wide range of colors and substantial hardness (Mohs 7.5), zircon has served as a gemstone since antiquity. Some biblical scholars suggest that zircon was among the 12 gemstones in the sacred breastplate of the high priest of the Israelites that dates to 1450 B.C. In later medieval times, zircon was thought to promote sleep and bring prosperity, honor and wisdom to its wearers.

The most common of zircon’s many colors is brownish-red. Yet it was blue zircon that enjoyed great popularity in Victorian-era jewelry. By 1900, colorless zircon had become the preferred diamond simulant, thanks to its high index of refraction (1.92-1.96) that gives properly cut zircon gems a fiery, diamond-like glitter. To add to zircon’s gemological credentials, several large, reddish stones are included in the British Crown Jewels collection. Zircon is the birthstone for December.

Nevertheless, as a gemstone zircon does have an image problem. Unlike ruby and emerald, it lacks a dominant identifying color. And because of its previous role as a diamond simulant, zircon is still associated with imitation. Furthermore, zircon is often confused with the modern diamond simulant cubic zirconia (CZ), which is not zircon at all, but a stabilized, synthetic zirconium oxide.

Zircon and Uranium

Because of a mutual chemical affinity, zircon usually contains traces of the radioactive element uranium. The uranium atom’s unstable nucleus undergoes continuous atomic decay, emitting ionizing energy as particles and rays while disintegrating into a series of unstable isotopes that eventually ends with the stable form of lead.

Uranium’s radioactivity causes some zircon crystals to undergo metamictization, which occurs when radiation displaces electrons within the zircon lattice to slowly degrade the crystal structure. The effects of metamictization include rounded crystal edges; curving faces; separating cleavage planes; color alteration; and decreased hardness, density and transparency.

Metamictization can sometimes degrade transparent zircon crystals into opaque, amorphous masses. However, metamictization is not a problem with zircon gems. Radiation levels are far too low to be hazardous and the physical effects of metamictization in zircon become apparent only after millions of years.

The “Clock in the Rock”

The marriage of zircon and uranium makes possible very long-term radiometric dating. Rates of atomic decay are expressed in terms of “half-life,” the time in which radioactive isotopes lose 50 percent of their radioactivity. The uranium-238 isotope has the longest half-life at 4.5 billion years.

When igneous rocks solidify from magma, uranium is often present in their compositional minerals. Because atomic disintegration progresses at known, precise rates, measuring the ratios of the decay-chain isotopes reveals the extent of atomic decay and thus the time that the mineral crystallized.

To date extremely old materials, uranium must be trapped within an extraordinarily durable, abundant and widely distributed crystal such as zircon. Zircon is inert and insoluble, while its crystal structure assures survivability even in the high pressures associated with geological upheaval and burial. It’s very high melting temperature enables zircon to endure even the extreme temperatures of high-grade metamorphism.

Zircon can survive billions of years of physical and chemical weathering, transport, and redeposition, while its traces of uranium steadily tick away as the clock in the rock.

The Hadean Time Capsule

Geophysicists have radiometrically dated tiny zircon crystals from Western Australia’s Jack Hills to 4.404 billion years. These crystals formed during the Hadean Eon (4.6-4.0 billion years ago), the earliest and least understood of the Earth’s geological time divisions. The Hadean Eon was named decades ago when scientists believed that volcanoes and lava fields dominated the primordial Earth and created “hell-like” conditions unsuitable for life.

But data obtained from the Jack Hills zircon crystals paints a radically different picture of the Hadean landscape, one where the Earth’s crust formed much earlier than previously thought. Furthermore, the ratios of oxygen isotopes trapped within these zircon crystals indicate that an atmosphere and oceans already existed and that life was therefore already possible.

The Jack Hills zircon crystals were originally part of a granitic rock that completely weathered away. These crystals then mixed with sediments that lithified much later and metamorphosed into the 3.3-billion-year-old quartzite-conglomerate formation that geologists call the Jack Hills Quartzite. Polished slabs of this orange-brown meta conglomerate with their tiny reddish zircon crystals are now sold at gem and mineral shows and online.

So while diamonds might be “forever” in matters of love, when it comes to the radiometric dating of the early Earth’s oldest known crustal fragments, it seems that what is forever are zircons.

This story about what is zircon previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Steve Voynick.

The post What is Zircon? first appeared on Rock & Gem Magazine.

Gemstones of the Bible – Sacred Breastplate

The Bible’s most celebrated – and debated – reference to gemstones regards the sacred breastplate of the high priest of the Israelites, also known as “Aaron’s breastplate” and the “breastplate of judgment.” Described in detail in the Old Testament’s Book of Exodus, this golden breastplate was set with 12 different gemstones arranged in four rows of three gemstones each. Each gemstone was identified in ancient Hebrew, the original language of the Old Testament.

But the text of the original Hebrew Bible and the meanings of many ancient Hebrew words are now largely lost. Our knowledge of the Old Testament as presented in the Bible’s many English versions is based on 2,500 years of scholarly interpretation of Greek, Aramaic, and Latin translations

Gemstones of the Bible – Debating Identities

Not surprisingly, the identities of the breastplate gemstones have become confused. Modern English versions of the Bible collectively offer more than 40 different identities for the 12 breastplate gemstones. Most are modern names of gemstones, minerals, and mineral varieties, along with some archaic English names and several untranslated Greek and Latin names.

Adding to the confusion, modern artistic depictions of the breastplate often disregard the probable color and transparency of its gemstones. Many depict the gemstones as faceted, transparent gems, even though faceting as we know it today was not developed until about 1400 C.E. Prior to the first century B.C.E., most gemstones were opaque or translucent and were fashioned as cabochons.

For centuries, historians, theologians, and scholars have debated the identities of the breastplate gemstones and agree only on the general historical background of the breastplate itself. According to biblical scholars, the Old Testament was written over a period of 1,000 years, roughly from 1400 to 400 B.C.E. The breastplate was created about 1450 B.C.E. during the time of Moses. The Book of Exodus, which contains the breastplate description, is based almost entirely on oral tradition and was written in stages between 600 and 400 B.C.E.

Gemstones of the Bible – Tricky Translations

Most interpretations and translations of the names of the breastplate gemstones were provided by scholars with little, if any, geological, gemological, mineralogical, or sometimes even historical, awareness. Their translations are based largely on tradition, limited gemstone knowledge, personal whim, or simple phonetics – sapphieros must mean “sapphire,” and topazos must means “topaz.”

But Dr. James A. Harrell, Professor Emeritus of Geology at the University of Toledo, has taken a different approach to identify breastplate gemstones. A specialist in the archaeological geology of Egypt and the Middle East, Harrell presents his ideas in a paper published in the Bulletin for Biblical Research and titled “Old Testament Gemstones: A Philological, Geological, and Archaeological Assessment of the Septuagint.”

The Septuagint is a third through first-century B.C.E. Greek translation of the original Hebrew Bible. The name “Septuagint” stems from the Latin septu gint, meaning “seventy” and refers to the number of Jewish scholars who worked on the translation. As a first-generation translation, the Septuagint is the most direct linguistic link to the identities of the breastplate gemstones.

Gemstones of the Bible – Breastplate Order

In his research, Harrell considered all Septuagint passages that mention gemstones and not just those related to the breastplate. He also consulted numerous other contemporaneous ancient texts that describe gemstones that are likely the same as those in the breastplate.

Historically, Harrell considered the gemstones that were known to be in use in the greater biblical region (southwestern Asia, Egypt, and the eastern Mediterranean) during the first millennium B.C.E. He also applied geological criteria to gemstone identification and drew upon his own field research and personal examination of ancient gemstones in museum collections.

As described in Exodus, the order of the breastplate gemstones progresses from right to left, as does ancient Hebrew writing. The first stone in each row, therefore, appears at the right and the third stone in each row at the left. In the following discussion, the stones are identified by the transliteration of their Septuagint Greek names that have so confused translators.

Sardion

Sardion is the first stone in row one of the breastplate. It has been translated as “carnelian,” “sard,” “sardonyx,” and “red jasper.” Archaeological recoveries indicate that carnelian and sard, both translucent forms of microcrystalline quartz, were the most common gemstones throughout the biblical region during the first millennium B.C.E. Carnelian is reddish; sard is brownish. Sardonyx is a brown-and-white-banded type of sard. Red jasper, an opaque form of microcrystalline quartz, also served as a gemstone, but not nearly to the extent of carnelian.

In his Naturalis Historia, the Roman scholar Pliny the Elder (Gaius Plinius Secundus, 23-79 C.E.) describes sardion as a widely used, “fiery, red gemstone.” As a gemstone for the breastplate, Harrell concludes that bright-red carnelian would certainly have been chosen over sard, sardonyx, or red jasper.

Topazos

Topazos is the second stone in row one of the breastplate. Topazos has been translated as “topaz,” “chrysolite,” “emerald,” and “peridot.”

Its earliest reference, written in the second century B.C.E., describes “a delightful, transparent stone similar to glass and with a wonderful golden appearance.” Another calls it “topazion Ethiopias,” meaning “topazos from Ethiopia.” During the biblical period, “Ethiopia” referred to Egypt’s Eastern Desert and nearby Red Sea islands.

Pliny writes that this stone came from the Red Sea island of Topazum (now Zabargad Island). He calls it the largest of the precious gemstones and the only one that is affected by an iron file – a description that indicates topazos is peridot, the gem variety of the olivine-group mineral forsterite (magnesium silicate). At Mohs 6.5, peridot is just soft enough to be scratched with an iron file, unlike the harder emerald and quartz gemstones. And the basalt formations of Zabargad Island, a classic peridot locality, have yielded very large peridot crystals.

Topaz, or basic aluminum fluorosilicate, is much harder than peridot and does not occur in basalt. Topaz was given its modern name in the 18th century when it was confused with the ancient topazos. Harrell is confident that the breastplate’s topazos is definitely peridot.

Smaragdos

Smaragdos is the third stone in row one of the breastplate. Smaragdos has been translated as “beryl,” “carbuncle,” “emerald,” “malachite,” and “turquoise.” In his On Stones, the Greek scholar Theophrastus (ca. 371 – ca. 287 B.C.E.) writes that smaragdos refers to a group of bluish and greenish stones and that it is “good for the eyes,” implying a cool, soothing color. He also mentions that blocks of smaragdos large enough to fashion into obelisks were common.

Emerald was not readily available until mines in Egypt’s Eastern Desert opened in the late first century B.C.E. At that time, smaragdos was also referred to as emerald, probably because of its similar green color. But the size of the smaragdos that Theophrastus describes certainly does not indicate emerald.

Some early descriptions of smaragdos would fit malachite. During the first millennium B.C.E., malachite was mined as the primary ore of copper on Cyprus and the Sinai Peninsula, and in Israel’s Timna Valley. Malachite was associated with such colorful, oxidized copper minerals as turquoise, azurite, and chrysocolla, which sometimes occurred in large, intermixed blocks.

Anthrax

Anthrax is the first stone in row two of the breastplate. Anthrax has been translated as “carbuncle,” “emerald,” “ruby,” “turquoise,” and “red garnet.” The Greek word anthrax refers both to hot embers and to a gemstone with a similar, glowing red color. Theophrastus describes it as “very rare and small, and carved into signets,” and compares its color when held against the sun to that of a glowing, red coal. He notes anthrax being “angular and containing hexagons.” Garnet group minerals, which crystallize in the cubic system, often occur as spheroids with hexagonal faces.

Pliny, who refers to anthrax as carbunculus, notes its “exceptional brilliance.” The substantial density of garnet-group minerals produces a high index of refraction and thus greater “brilliance” than many other red gemstones. Pliny also observes an amethyst-violet tone in the basic red color of anthrax. The almandine-pyrope garnet series, which has purplish-red colors, were the garnets mainly used in antiquity. Although garnet was occasionally found in the biblical region, most came from India after the third century B.C.E. Harrell concludes that the breastplate’s anthrax is red garnet, most likely a member of the almandine-pyrope garnet series.

Sappheiros

Sappheirosis the second stone in row two of the breastplate.

Sappheiros, the origin of our modern word “sapphire,” has been almost universally translated in the Bible as “sapphire.” Yet sappheiros is actually lapis lazuli, a prized gemstone and a major trading commodity throughout the biblical period.

Lapis lazuli is a metamorphic rock consisting of lazurite, calcite, pyrite, and other minerals. Lazurite, a basic sodium calcium aluminum sulfate chlorosilicate, is the primary mineral in lapis lazuli and the cause of its striking blue color. In top-quality lapis lazuli, pyrite appears as glittering, disseminated specks. Many ancient writers have compared the dark-blue color and glittering pyrite specks of sappheiros to a star-filled night sky.

Since 4000 B.C.E., the Sar-e-Sang mines in northeastern Afghanistan have produced the world’s finest lapis. The corundum gemstone we now know as sapphire was not available in the first millennium B.C.E. Had it been available, its extreme hardness would have made it very difficult to work.

There is no doubt that the breastplate’s sappheiros is not sapphire, but lapis lazuli.

Iaspis

Iaspis is the third stone in row two of the breastplate.

The Bible’s long list of iaspis translations include “beryl,” “diamond,” “rock crystal,” “emerald,” “jasper,” “onyx,” “moonstone,” “chrysoprase,” and “amazonite.” Theophrastus writes that iaspis was carved into seals, and groups it with smaragdos, implying that it has a bluish or greenish color.

Pliny describes it as “a highly prized stone,” translucent and with blue and green varieties. Although these descriptions are general, iaspis could be greenish microcrystalline quartz, perhaps a color variation of jasper. Iaspis is also the origin of the modern word “jasper.”

But another possibility is amazonite, the green-to-blue variety of microcline feldspar, which was mined in Egypt during the first millennium B.C.E. and saw limited use as a gemstone. However, Harrell believes that iaspis is more likely a greenish microcrystalline quartz, perhaps a form similar to chrysoprase.

Ligyrion

Ligyrion is the first stone in row three of the breastplate.

Although ancient literature consistently indicates that ligyrion is amber, a fossilized tree resin, it has also been translated as “zircon,” “tourmaline,” and “opal.” The ancient Greeks knew ligyrion as elektron and were aware of its electrostatic properties. Rubbing ligyrion with wool cloth produces a strong negative electrostatic charge that attracts feathers and other light, positively charged materials.

Theophrastus writes that elektron is found in Liguria, an area of northwestern Italy and southeastern France, where it is “dug from the earth” and “has the power of attraction.” Other writers use the words elektron and ligyrion interchangeably.

In his Naturalis Historia, Pliny notes the sources of ligyrion, which he calls sucinum, as Liguria and the “northern sea” – the latter referring to the Baltic Sea coast. The Baltic coast supplied the Roman Empire with large quantities of amber and remains the world’s most prolific amber source.

Some biblical historians have translated ligyrion as yellowish or brownish zircon; others as tourmaline, likely because of tourmaline’s electrostatic properties. But neither zircon nor the tourmaline-group minerals were used as gemstones during the first millennium B.C.E., whereas amber was common. Harrell believes that ligyrion is definitely amber.

Achates

Achates is the second stone in row three of the breastplate.

Achates has usually been translated as “agate,” which is almost certainly correct. In On Stones, Theophrastus discusses achates as “a handsome stone from the river Achetes in Sicily that fetches a high price.” The Achetes River (now the Drillo River) is the root of the English word “agate” and a classic agate locality.

Pliny describes different colors and patterns of achates, all of which fit agate. He also writes that achates “was once held in high esteem, but now enjoys none,” apparently indicating that formerly valuable translucent and opaque gemstones had fallen out of favor in Rome by the first century C.E. and had been replaced by transparent stones from India.

Amethystos

Amethystos is the third stone in row three of the breastplate. (row three, third stone)

The Greek word amethystos, the root of the English word “amethyst,” has been translated only as amethyst and has no conflicting identifications. Amethystos means “without drunkenness;” the stone was believed to prevent drunkenness or to alleviate its unpleasant aftereffects.

Theophrastus discusses amethystos as “transparent … with the color of red wine … and found by splitting certain rock.” This description fits amethyst, because red wine is actually purplish-red, and amethyst often occurs in geodes that must be “split.” Pliny describes the stone as “violet” and notes that it comes from Egypt, where the Abu Diyeiba mine produced amethyst throughout the first millennium B.C.E. The ancient descriptions of amethystos can only fit amethyst.

This story about gemstones of the Bible previously appeared in Rock & Gem magazine. Click here to subscribe! Story and photos by Steve Voynick.

The post Gemstones of the Bible first appeared on Rock & Gem Magazine.