Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Nani Burns
Garnets are a group of silicate minerals used since the Bronze Age as gemstones and abrasives. They fall into the nesosilicate category .The word Garnet comes from the 14th century middle English word Gernet which means dark red. This is derived from Latin granatus from granum which means grain or seed. Garnets are most popularly known throughout the United States as the birthstone for the Month of January. They come in many different colors; red, orange, yellow, green, purple, brown, blue, black, pink, and colorless; though the reddish hues are the most common and blue being the most rare. The garnets lights transmission properties vary from one gemstone to the next; they range from transparent to opaque. Opaque stones are used for industrial purposes as abrasive materials. Likewise, their luster also ranges from either vitreous or glass-like to resinous or amber-like. A strong pick up response to a neodymium magnet; a magnet made from an alloy of neodymium, iron, and boron: these are the strongest types of magnets available commercially; this response sets garnets apart from other natural transparent gemstones. Garnets range from a 6.5 to a 7.5 on the Mohs scale of hardness due to the different chemical composition of each of the six types of garnet. The six types of garnets are: Almandine, pyrope, spessartine, grossular, uvarovite, and andradite.
Red garnets were the most commonly used gemstone in the late Antique Roman World. They were inlaid into gold cells, this style was called Garnet Cloisonne. This style could be found from Anglo-Saxon England to the Black Sea. Pure garnet crystals are still used in today’s society. Along with being the birthstone for January garnet is also the state mineral for Connecticut, the gemstone for New York, and the star garnet is the gemstone for Idaho.
Almandine is an iron garnet, deep red to purple in color. It is a 7-7.5 on the Mohs scale. Almandine is found rather abundantly in Sri Lanka and is sometimes called the Ceylon Ruby. When the color runs more to the violet it is sometimes called a syriam garnet.
Pyrope is the only garnet that is always red when in its
natural color. This is how it got its name which is Greek for fire and eye.
This garnet is less common than most other garnets, however is still a widely
used gemstone. It has a hardness of 7-7.5 on Mohs scale. Pyrope can be found in
ultramafic rocks, typically peridotite.
Spessartine is found in Bavaria, Germany. Most often in
granite pegmatite. It can also be found in Australia, Myanmar, India,
Afghanistan, Israel, Madagascar, Tanzania, and parts of the U.S.A. It is an
orange yellow in color which has earned it the name the mandarin garnet. Some
violet/red variations can be found in rhiolites within the U.S. These garnets
have the full range of 6.5-7.5 on the Mohs scale.
Grossular garnets range from green to cinnamon brown, red, and
yellow in color. They fall from. 7-7.5 on the Mohs scale. These garnets are
found in metamorphosed Limestone. He green Grossular from Kenya and Tanzania
known as Tsavorite is a highly sought after variety of garnet gem.
Uvarovite is a green garnet that is one of the rarest. It is
found in Spain, Russia, and Quebec. It is associated with chromium ores. It is
also found in Finland, Iran, Norway, and South Africa. It ranges from a 6.5-7.5
on Mohs scale.
The sixth variant of garnet, Andradrite, can be found in black,
vivid green, or yellow green coloring. It can be found in metamorphosed
limestone in Italy, Russia, Arizona, California, and the Ukraine. It has a
hardness of 6.5-7 on Mohs scale.
With its versatile uses from as a gemstone in jewelry to being ground up and being used industrially as an abrasive powder, with its rainbow of colors, with six different species found all over the world, the garnet definitely shows its versatility as a precious gem.
References: Wikipedia USA
Roger Weller skywalker.cochise.edu for some images
Google Images for images of Pyrope and cluster of Garnets