January

Garnet has been the birthstone for January since the 15th century, at least. With a Mohs hardness of 6.5 to 7.5, it can be faceted into beautiful gemstones that wear well in jewelry. Since the term “garnet” actually refers to a group of nesosilicate gems, those born in this month can choose from a rainbow of colors.

The most common members are red almandine, an iron-aluminum silicate; red pyrope, a magnesium aluminum silicate; orange-yellow spessartine, a manganese aluminum silicate; the yellow or green varieties of andradite, a calcium-iron silicate; predominately green grossular, a calcium-aluminum silicate; and rare, bright-green uvarovite, a calcium chromium silicate.

February

From the 15th century to the present, amethyst has been the preferred birthstone for February. Amethyst belongs to a mineral family that can compete with garnet for diversity of color: quartz.

Pure quartz is colorless, as exemplified by Herkimer diamonds. The causes of amethyst’s shades of pale violet to rich purple are radiation and the inclusion of iron impurities and trace elements.

As a rule, amethyst crystals are short and stubby, and occur in large numbers, often filling a large vug a hollow petrified tree section, or lining the inside of a geode. Fine crystals that are large enough to produce a faceted gem of over 20 carats are rare.

March

The current choice of a birthstone for March is aquamarine. Aquamarine is a variety of beryl (Mohs 7.5-8). Its name was derived from the fact that the beautiful, transparent, blue-green coloration of the gem resembles that of seawater. It can be found in translucent to transparent crystals that form in the hexagonal system. The six-sided crystals are often striated lengthwise.

Aquamarine develops in metamorphic rocks and, more often, in pegmatites.

April

Before 1900, a person with an April birthday had two choices of birthstone: diamond or sapphire. During the 20th century, however, diamonds became the preferred stone.

Diamond, a mineral consisting of pure carbon, heads the list of all gemstones for its beauty and hardness. A 10 on the Mohs Scale of Hardness, it is resistant to scratching and is an ideal gem to set in rings. Its hardness results from the arrangement of its atoms in cubes.

All diamonds have slightly rounded faces, and they’re so smooth they feel greasy to the touch. They can be colorless and water clear to blue, pink, yellow, brown, green or black, and transparent or translucent. They shine with an adamantine luster when held to the light.

May

There were two choices for May birthstones for several hundred years: emerald and agate. The popularity of agate seems to have waned at the turn of the 20th century, so emerald is now the favorite. It’s the green member of the beryl family of gemstones. The color varies from bright green to pale green and, sometimes, darker shades of blue-green.

Fine emeralds have a velvety surface appearance and, in the better stones, an even distribution of color. One bad trait of emeralds is a tendency to have inclusions. It’s rare to find an emerald without some slight imperfection. This in no way deters from the beauty of this gemstone, though. It can also be one way of determining whether an emerald is a simulated gem or the real thing, as manmade stones have no imperfections.

June

The contemporary choices for June are pearl, moonstone and alexandrite. Of course, a pearl is the organic product of marine bivalves and not a mineral.

Moonstone is a variety of feldspar that shows adularescence, or schiller, an optical effect that produces a milky luster with a bluish tinge that appears to move across the stone when it is tilted. The phenomenon is named after the feldspar variety adularia.

Alexandrite is a color-change variety of chrysoberyl (beryllium aluminum oxide). This is a very rare and expensive gemstone. It has a hardness of 8.5, and its crystals are either tabular or prismatic. The distinction between alexandrite and chrysoberyl is simply color. A strange characteristic of alexandrite is that it is red, purple or violet when held under artificial light, but in daylight, it looks green.

July

Ruby is the standard birthstone for the month of July. It is a corundum (aluminum oxide) gem that gets its color from the presence of chromium in its structure. An exceptionally hard mineral, corundum illustrates a hardness of nine on the Mohs scale. “Pigeon-blood” red is the preferred color for rubies, though they also occur in lighter shades, including pink. All other colors of corundum are called sapphires.

Ruby exhibits all the desirable properties of a jewelry stone: beauty, durability, optical properties, and rarity. Some rubies display a star or asterism when fashioned into a cabochon. This effect is caused by the reflection of light from numerous inclusions of minute, needle-like crystals of rutile. Corundum crystallizes in the hexagonal system with a tabular-barrel-shaped habit.

August

Current birthstones for August are peridot, the gem-quality form of olivine and spinel. Olivine makes up a large portion of the earth’s mantle. Rocks containing olivine have been brought to the surface by volcanic action and actually blown out in the form of volcanic bombs. Masses of olivine have been found in meteorites, and the Apollo astronauts brought basaltic rocks back from the moon that contained olivine.

A popular jewelry stone, peridot has a hardness of 6.5-7 and can be transparent or translucent, with a vitreous luster. Its color shades from deep green to apple green, yellow-green or olive. It’s most often found in granular nodules, forming short, prismatic crystals in the orthorhombic system.

Spinel is the gem-quality member of the larger spinel group. Its hardness (Mohs 7.5-8.0) makes it ideal for jewelry use. Its spectrum of colors includes red, pink, purple, blue and lavender. In times past, red spinel was often mistaken for ruby. A notable example is the Black Prince’s Ruby, set in the royal crown of England.

September

The birthstone for September is sapphire. This term refers to any corundum (aluminum oxide) gem that has any color other than red (ruby). Sapphires may be colorless, blue, green, yellow, orange, brown, pink, purple, gray, black, or multicolor. At Mohs 9, its hardness is second only to that of a diamond.

Heat treatment is sometimes used to give natural blue sapphires a deeper, more pleasing color. Natural star sapphires, which display the optical phenomenon of asterism, are very rare.

October

Two options for October are opal and tourmaline. Opal is a magnificent gemstone with a play of color or “fire” in all colors of the spectrum. Spaces between the tiny spherules of silica that make up the gem diffract light into its spectral colors. Red, yellow, green and blue, in strong to pastel shades, flash from the stone when it is tilted.

Opal occurs in common and precious types. Common opal does not display any reflective fire. It may have a honey-yellow, brown, gray or colorless body color that is milky and opaque. Opal (Mohs 5-6) is not a very hard gemstone.

Tourmaline, a silicate of boron, has a complicated chemical composition, in which a number of elements, including calcium, iron, sodium and aluminum, may combine. It has a Mohs hardness of 7-7.5.

It belongs to the trigonal crystal system and its habit is hemimorphic (a crystal having two ends of an axes unlike in its planes).

Because of the coloration of the individual stones, tourmaline has several names, including schorl (black), rubellite (red), indicolite (blue), and dravite (brown). Tricolor crystals are common. The popular watermelon variety has an outer layer of green around a red core.

November

The current birthstones for November are topaz and citrine. People tend to think of topaz, a silicate mineral with aluminum and fluorine, as a yellow stone, but heat-treating and color-enhancing adaptations have made blue the predominant color on the market. It is an allochromatic mineral, which means its color is caused by internal defects in the crystal and has a Mohs hardness of eight.

Citrine is the golden member of the quartz family (silicon dioxide). Though quartz in its many forms is one of the most abundant minerals on earth, fine, gem-grade crystals are not that common. Citrine is affordable and, when faceted, rivals more expensive gemstones in beauty.

December

There are three birthstones for December: turquoise, blue zircon and tanzanite. Turquoise (hydrated copper aluminum phosphate) is an opaque, blue-to-green, massive gem material. It has a relatively low hardness of Mohs 5-6, so care must be taken with turquoise jewelry.

The rarest and most valuable variety is robin’s-egg blue with black “spiderweb” veins of limonite. Fake turquoise, consisting of dyed howlite or magnesite, is common. Buyer beware.

Zircon (zirconium silicate) can be blue, black, red, brown, green, yellow, smoky, or water-clear. It has an adamantine luster much like that of a diamond, and it is often misidentified as such.

Tanzanite, the blue/purple variety of zoisite (basic calcium aluminum silicate), is a recently introduced alternative for December. Tanzanite crystals in shades of yellow to brown, green, pink, gray or blue are often heat-treated to produce a gemstone that is a beautiful and permanent blue.

This story about what are birthstones by month previously appeared in Rock & Gem magazine. Click here to subscribe! Story by Kenneth H. Rohn.

The post What are the Birthstones by Month? first appeared on Rock & Gem Magazine.

The Lure Behind Spinel

Beyond the visual appeal, colored gems make life beautiful. They are associated with positive aspects of life whether a wedding, birthday or the crown jewels of royalty. Spinel is high on many blue gems and minerals lists.

Regardless of the elaborateness, or simplicity, they are the brilliance of something good. This quality is not lost on Wez Barber of Mahenge Gems, a wholesale company that strives to connect gem dealers and artists with the high-quality gemstones that he and his partner, Rimsan Nifal, acquire directly from artisanal mines in Tanzania and Sri Lanka.

While they handle many varieties of colored gemstones, spinel holds a favored place for Mahenge Gems. “I like the story of it being the underdog,” explained Barber. “It’s always been undervalued and misunderstood. There is a mystique behind it.”

Derived from the Greek word for “spark,” spinel certainly earns its moniker with its vibrant color and brilliance. For centuries it was initially mistaken as a ruby and only recognized as its own mineral in 1783. Many of the world’s most illustrious “rubies,” are, in fact, spinel.

While it’s found in royal collections throughout the world, including the Diamond Fund which is a part of Russia’s jeweled past. One of the most famous pieces is the 170-carat “Black Prince’s Ruby,” which is actually a blood-red spinel. It was originally given to Edward of Woodstock, the “Black Prince,” in 1366. By 1415, King Henry V wore it into battle and nearly lost it and his life during the conflict.

It was a favorite piece of King Henry VIII and his daughter, Queen Elizabeth I. It currently resides in Britain’s Imperial State Crown in the Tower of London.

For centuries, spinel has been an underrated stone, even when it shares many of the positive characteristics in color, vibrance, and hardness (Mohs Scale of Hardness value of eight) as more popular gems. “It’s one of the easiest stones to cut, and strangely enough, it’s very forgiving,” said Barber.

Where are Spinel Gems Found?

Depending on the percentage of chromium, iron, and cobalt influencing the stone, spinel is found in pinks, reds, and oranges to the hues of blues and violets. Found in Brazil, Kenya, and Vietnam, the Mahenge region of Tanzania is particularly well-known for this gemstone after the exciting discovery of the fiery pink spinel in 2007. But 2021 brought something completely different.

“In late October of last year this blue material came through the office doors,” said Barber. He said that Nifal immediately FaceTimed him to show him the new discovery. “He was so excited because of the color. It looks like a royal blue sapphire. He was like a kid in a candy shop with tears in his eyes.”

While there is a percentage of iron in the stone, Barber said, “It’s cobalt that gives it that electric blue. The color is stunning.”

This latest discovery is also larger than other cobalt spinel specimens found in other parts of the world. Barber said the material from Tanzania is coming out in two, three, and even four-carats in clear stones, which is undoubtedly exciting news for gem dealers.

A Business Built on Connections

Connecting high-quality gems, such as the cobalt spinel, with experts within the world of gemstones is the heart of Mahenge Gems.

“This business is entirely relation-based. It’s all about meeting people and doing business with people you like,” he said. “When we’re at the shows everyone gets together.”

This includes the partnership between Barber and Nifal who balance each other’s strengths and specialties. “(Nifal) looks after the sourcing. He can look at a piece of rough and know what is going to come from that stone,” said Barber. “I’m the guy who takes care of the marketing and the backend stuff.”

And for Mahenge Gems, the most important working relationships begin at the ground level with those who’ve mined these materials for generations. “Artisanal mining is very different. We have to look after the people in the supply chain,” he said. “We do what we can to give back. It can’t all be taken, take, take.”

The town of Mahenge was literally built on the mineral business, yet, in reality, this way of life is finite. Once these materials are extracted, they don’t grow back. Barber uses tanzanite as an example. The vibrant blue and purple-hued tanzanite exhibits pleochroism, the quality to display different colors depending on the angle of the stone, is unique to this region and, being identified in 1967, is one of the newest gems. Unfortunately, it’s not an unending source. “Tanzanite will be mined out in 20 years,” he said. With this knowledge, he feels it’s important to give miners a fair price within this mutually beneficial relationship.

Real-World Value

The other aspect of the gemstone industry that Barber appreciates is they are handling real, hard assets with a market value. This is not a fiat currency and there is nothing digital about holding a gemstone in your hand.

Whenever traditional markets waiver, hard assets surge ahead. “The stone market in gems is very strong,” explained Barber, who noted that people who are moving off the stock market are pushing prices of stones to record highs. Even with multiple supply chain issues, gem sales are strong. “In Sri Lanka, the price of sapphires has doubled in the last six months,” he said. And while he doesn’t expect it to last forever, it’s indicative of the appreciation and value of these high-quality gemstones.

When looking at the world of precious and semi-precious stones, diamonds have long received the glory, but is it well-earned?

As Barber points out, “Diamonds are judged on the absence of everything. Whereas colored gemstones are judged on the presence of everything.” From the local miners to the person who wears the finished jewelry, there is a presence of life in the stones brought to the world by Mahenge Gems.

This story about spinel gems previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Amy Grisak. Sponsored by Mahenge Gems.

The post Where are Spinel Gems Found? first appeared on Rock & Gem Magazine.

Gold panning, the simplest form of hydraulic gravitational separation, relies on differences in mineral density, which is measured in specific gravity. The specific gravity of quartz, the primary component of most sands and many rocks, is 2.65. That of native gold is most often between 17.0 and 18.0. Because of gold’s far greater density, it remains in the pan while the common, quartz-based gravels are washed away.

Most Common Concentrates

Most gold-pan concentrates consist of relatively dense, iron-based minerals such as magnetite (iron oxide, ferrous-ferric), hematite (iron oxide, ferric), ilmenite (iron titanium oxide), and chromite (iron chromium oxide), all of which have substantial specific gravities. between 4.3 and 5.3. Their generally dark colors are the origin of the term “black sand.”

With careful panning techniques, minerals with specific gravities as low as 2.9 will remain in the pan concentrate.

These minerals can include everything from the sulfides, oxides, and carbonates of heavier metals to such relatively dense gemstones as diamond, ruby and sapphire (corundum), topaz, garnet, spinel, and chrysoberyl.

Concentrates Aid In Deposit Discovery

Both the historic silver discoveries at Nevada’s Comstock Lode and Leadville, Colorado, were made by gold miners who identified oxidized silver minerals in their pan concentrates. Gold panners also discovered Montana’s five major sapphire deposits.

Sometimes pan concentrates can be a big problem. I once sluiced gold-bearing gravels in an Alaskan creek—where the pan concentrates consisted largely of tiny bits of native lead—from which the gold particles could be separated only by amalgamation.

Even in this age of high-tech mineral exploration, panning remains a valuable prospecting tool for many minerals other than gold. In the late 1980s in northern Canada, pan concentrates led to the discovery of diamond-bearing kimberlite pipes.

Prospecting for ‘Indicator Minerals’ Pays Off

When exploration geologist and prospector Chuck Fipke panned his way across 400 miles of tundra, he was not searching for diamonds per se, but for the “diamond-indicator” minerals that typically associate with diamonds in kimberlite environments, but are much more abundant and readily identifiable.

Fipke was specifically looking for black ilmenite (iron titanium oxide), red pyrope garnet (magnesium aluminum silicate), and green chromium-rich diopside (calcium magnesium silicate), which all have sufficient densities to be retained in pan concentrates.

During several years of prospecting, Fipke never panned a single diamond. He did, however, follow a trail of ilmenite, pyrope, and diopside. He eventually panned a green diopside crystal with no alluvial wear at all—enough to convince him that he was standing atop the eluvial remains of weathered kimberlite pipe. Core drilling revealed a kimberlite pipe that has since been developed into the billion-dollar Ekati diamond mine.

Along with the common black sands, you’ll find an array of other minerals with densities mostly in the 3.0-5.0 specific-gravity range.

Under a loupe, the combination of colors can be eye-catching. The common garnet-group minerals impart bright orange, pink, and red hues. Some concentrates even have a “Christmas tree” appearance when garnets mix with another common, dense mineral—green epidote (basic calcium aluminum iron silicate).

Panning Leads to Fascinating Discoveries

Although many minerals in gold-pan concentrates are abraded and rounded from alluvial wear, some retain enough of their original crystal forms to aid in identification.

So gold pans are not just gold-recovery tools, but geological sampling instruments. Take a closer look at those ubiquitous “black-sand” concentrates and you’ll be pleasantly surprised at the cornucopia of interesting minerals that meet the eye.

This story about gold pan concentrate appeared in Rock & Gem magazine. Click here to subscribe. Story by Steve Voynick.

The post What are Gold-Pan Concentrates? first appeared on Rock & Gem Magazine.

Spinel Characteristics

While many collectors prefer the luscious red variety, spinel also can be black, green, blue, lilac, orange-brown, pink or white. All have a glassy luster and are very hard with a Mohs ranking of 7.5 to 8.

Red and lilac varieties may fluoresce, such as those from the Sterling Hill Mining Museum, a brilliant red or yellow-green under ultraviolet light. Spinel is one of a series of closely related mineral varieties that depend on the amount of magnesium present.

If the magnesium is wholly or partly replaced by iron, the result is a mineral called hercynite; if replaced by zinc, the result is gahnite; and if replaced by manganese, the result is galaxite.

Spinel’s Similar Differences

Spinel is often found in such metamorphic rocks as calcareous gneisses and marbles (sometimes in association with ruby corundum), as well as in plutonic rocks such as gabbro. It’s also found eroded from its host rock in alluvial (river gravel) deposits.

In the United States, spinel is common in the metamorphosed limestones of the New York-New Jersey highlands belt. Myanmar, Sri Lanka and Madagascar provide significant amounts of gemmy spinel for use as gemstones to be set into jewelry.

A famous crown jewel of Great Britain, the Black Prince’s Ruby, is actually spinel! Like corundum, spinel can be synthesized by what’s called the Verneuil method, and many common synthetic gemstones are spinel. By adding different pigmenting elements, labs have been able to produce varied colors, thus allowing for spinel to be used to synthesize lookalikes for such gemstones as sapphire, aquamarine or peridot.

This story about spinel previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Jim Brace-Thompson.

The post Getting to Know Spinel first appeared on Rock & Gem Magazine.