Gem Chat: January's Birthstone - the Diverse, Prized, and Historically Significant Garnet

January 3, 2014

The Garnet Group

Garnet, January’s birthstone, is one of the most diverse gemstones. Garnets are not a single gem type, but rather a wide variety of closely-related minerals that form the Garnet Group.

All garnets have essentially the same crystal structure, but they vary in chemical composition, and as a result, garnets vary widely in appearance, most notably color. Garnet actually displays the greatest variety of color of any mineral, occurring in every color, including red, pink, orange, yellow, green, purple, and very rare blue and colorless varieties.

While gemologists break down the Garnet Group of Gems into more than 20 garnet species, many garnets are actually chemical mixtures of two or more garnet species, and only a few species are commercially important gems. The major five garnet gem species are: pyrope, almandine (also called almandite), spessartine, grossular (grossularite), and andradite. The sixth important garnet gem species is uvarovite - a green garnet that usually occurs as crystals too small to cut, but when large enough to fashion, uvarovite is sometimes set as clusters in jewelry.

Pyrope and almandine garnet species range in color from purple to orangy red. Spessartine garnet is found in a variety of orange colors. Grossular garnet has perhaps the widest color range of any garnet species, from colorless through yellow to reddish orange and orangy red, to a strong, vibrant green.

Gemologists further break down garnet species into garnet varieties, usually depending on color. For example, the Andradite species of garnet comes in yellow to green colors. The bright green variety of andradite is called demantoid garnet, and fine demantoid specimens are highly prized by gem collectors.

Both tsavorite and hessonite garnets are varieties of the grossular garnet species. Tsavorites are green, while hessonites range from orange and orangy red to brownish red.

Rhodolite is a beautiful purplish red variety of garnet and a very popular gemstone in designer jewelry.

Red garnet is one of the most common and widespread of gems, found in metamorphic rocks (rocks altered by heat and pressure) on every continent. Not all garnets are as abundant as the red ones. The green tsavorite garnet also occurs in metamorphic rocks, but it’s rarer than almandine garnet because it needs unusual rock chemistries and special conditions to form. The famous green demantoid garnet is also a rarer variety of garnet.

Besides brilliance and attractive colors, garnets sometimes exhibit optical phenomena such as asterism (a star-shaped pattern or horsehair-type reflections), chatoyancy (a “cat’s-eye” seen in reflected light), or a color change when viewed under different types of lighting similar to the rare gemstone alexandrite.

Red Almandine Garnets

While garnet comes in an extraordinary range of color, it is most commonly known for its rich shade of red. The deep red transparent stones we normally associate with gemstone garnets are typically from the Almandine species, and this particular garnet has been appreciated throughout history since biblical times.

Garnets have been used since the Bronze Age as gemstones and abrasives. When worn, garnet is thought to bring one a sense of calm. The word "garnet" is derived from the 14th century Middle English word "gernet" meaning "dark red." The name garnet is also associated with the pomegranate fruit, because the more commonly found deep red varieties of the gemstone resemble pomegranate seeds.

The term "carbuncle" was often used in ancient times to refer to red garnets, although it was used for almost any red stone. Carbuncle was thought to be one of the four precious stones given to King Solomon by God.

Thousands of years ago, red garnet necklaces adorned the necks of Egypt’s pharaohs, and were entombed with their mummified corpses as prized possessions for the afterlife. Centuries later, in Roman scholar Pliny’s time (23 to 79 AD), red garnets were among the most widely traded gems, and in ancient Rome, signet rings with carved garnets were used to stamp the wax that secured important documents. Red garnets were the most commonly used gemstones in the Late Roman world, and of the “barbarian” peoples who took over the territory of the Western Empire.

In the Middle Ages (about 475 to 1450 AD), red garnet was favored by clergy and nobility. Red garnets were inlaid in gold cells in the cloisonné technique, a style called garnet cloisonné in Anglo-Saxon England. Red garnet’s availability increased with the discovery of the famous Bohemian garnet deposits in central Europe around 1500. This source became the nucleus of a regional jewelry industry that reached its peak in the late 1800s.

Garnets are still one of the most important gems used in fashion and designer jewelry.

Garnet Properties

Hardness and Toughness

Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, the garnet mineral group shows a range of hardness of about 6.5 to 7.5 on the Mohs scale. For example, almandine, pyrope, spessartine, and tsavorite garnets are on the harder side, while demantoid is slightly softer.

Garnets have fair to good toughness, making them durable enough for all jewelry styles as long as they are treated with the proper care. Garnets should not be subjected to any hard blows or rough wear.

Stability

Garnets are stable to light and chemicals, although they can be attacked by hydrofluoric acid.

Clarity

Typical garnet clarity depends on garnet type. Red gem-quality garnets are known for their high transparency and clarity. Gem-quality red garnets including almandine, pyrope, and rhodolite typically do not have eye-visible inclusions. Some of the orange garnets, like spessartine and hessonite, often have eye-visible inclusions. Grossular garnet is typically translucent, making it popular for cabochons, beads, and carvings.

Cut

Many garnets are cut into standard shapes and standard sizes to allow easy setting into jewelry. This is especially true of many red garnets. Red garnets also are classic materials for cutting into cabochons and beads. Garnets are also popular gemstones used for designer cuts and carvings. Expensive garnets like fine-quality tsavorite are cut into shapes and cutting styles that allow more of the weight to be retained from the rough. Demantoid garnet is often cut to exacting proportions that allow the best possible display of its fire.

Carat Weight

Garnets can be found in all sizes and weights. Some garnets like demantoid and tsavorite are more commonly found in small sizes, so their value goes up significantly with size. Other garnets like almandine are far more common in larger sizes so there’s no dramatic rise in value as size increases.

Recommended Care of Garnet Jewelry

Garnets have been among the most popular gemstones in jewelry throughout history, and they can provide a lifetime of enjoyment with proper care.

Since garnet is not the hardest of gemstones, the safest way to clean is just with warm soapy water.

Ultrasonic cleaning is usually safe unless the garnet has inclusions or fractures.

Steam cleaning garnet is risky and not advised.

Treatment and durability considerations: Garnets might rarely be treated by fracture filling. Only warm soapy water should be used to clean fracture-filled stones.

Arpaia Lang Gem Quality Red Garnet 14kt Stud Earrings $180