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Birthstones
Michael Schweska
Physical Geology
Spring 2005

                              
 

                                         January Birthstone-Garnet

 
The gemstone Garnet is the official birthstone for January as adopted by the American National Association of Jewelers in 1912. Garnet may be given as the anniversary gemstone for the second and sixth year of marriage. It is also the stone for the Zodiac sign Aquarius. Garnets range from a 7.0 to 7.5 on the Mohs scale of hardness and are found in the U.S. (Arizona), South Africa, Argentina, Australia, Brazil, Myanmar (Burma), Scotland, Switzerland and Tanzania. Click here for a picture in natural form. Garnets Click here for a picture in natural form. Click here for more faceted and natural specimens.

Description:

Garnets as a group are relatively common in highly metamorphosed rocks and in some igneous formations. They form under the high temperatures and/or pressures that those types of rocks must endure. Garnets can be used by geologists as a gauge of how much temperature and pressure the rock has endured. As a gemstone, garnets have had a mixed reputation. Garnets do possess high indices of refraction, are hard enough, have pretty colors, are wonderfully transparent, lack cleavage and are durable; thus making good candidates for gemstones. Garnet derived its name from the Latin word granatus, meaning like a grain and also the color of the pomegranate seed. Grain refers to the mode of occurrence wherein crystals resemble grains or seeds embedded in the matrix. Garnet is a family of minerals having similar physical and crystalline properties. There are a number of trade and variety names for garnet, most of these names are for particular colors of a specie.

Chemistry:

The general formula for most of the garnets is A3B2(SiO4)3. The A represents divalent metals such as calcium, iron, magnesium and/or manganese. The B represents trivalent metals such as aluminum, chromium, iron and/or manganese and in the rarer garnets; vanadium, titanium, zirconium and/or silicon. The general formula for a couple of rarer garnets (hibschite and katoite) is A3B2(SiO4)3-X(OH)4X. The main differences in physical properties among the members of the garnet group are slight variations in color, density and index of refraction. All species of garnet have been used as gemstones.
  • Grossular (also known as grossularite), Ca3Al2(SiO4)3, Calcium Aluminum Silicate has many color possibilities and is probably the most colorful of the garnets. It is believed that these garnets form from the metamorphism of impure siliceous limestones. Grossular can be colorless, white, gray, yellow, yellowish green, various shades of green, brown, pink, reddish, or black. Hessonite is the variety name for a fine orange, cinnamon brown, or pinkish variety of grossularite. Tsavorite is the trade name for fine dark green grossularite.
  • Almandine (also known as almandite), Fe3Al2(SiO4)3, Iron Aluminum Silicate is the most common of the garnets and is usually the garnet found in garnet schists (a type of metamorphic rock composed mostly of mica). Pure almandine and pure pyrope are rare in nature and most specimens are a percentage of the two. Almandine is deep red, brownish red, brownish black or violet-red in color.
  • Pyrope, Mg3Al2(SiO4)3, Magnesium Aluminum Silicate is the only garnet that is always a shade of red. Although less common than most other garnets, pyrope is a common gemstone. Pyrope is the only garnet whose most common source is igneous rather than metamorphic. Most pyrope comes from ultramafic igneous rocks that contain olivine and/or diamond. Metamorphic pyrope comes from the metamorphism of the igneous rocks previously mentioned or from magnesium rich rocks. Almandine and Pyrope form a series in which iron substitutes for the magnesium in pyrope. In fact, pure Pyrope is unknown in nature and the various proportions are referred to as pyrope-almandine mixes. One mixture of approximately two to one (pyrope to almandine) is a variety called Rhodolite which has an attractive red-lavender color and is cut as a gemstone. Pyrope is commonly purplish red, orangy red, crimson, or dark red. Pyrope is difficult to distinguish from Almandine but is usually clear and free from flaws, at least more so than almandine. Fine-quality Pyrope garnets from Czechoslovakia are often called Bohemian garnets.
  • Spessartite (also known as spessartine), Mn3Al2(SiO4)3, Manganese Aluminum Silicate like other garnets, forms rounded crystals with 12 rhombic or 24 trapezoidal faces or combinations of these and some other forms. This crystal habit is classic for the garnet minerals. Spessartine is formed in manganese rich metamorphic enviroments and in some granitic pegmatites. Spessartine is somewhat rare but occassionally will accompany other minerals and make a nice accessory mineral to an outstanding mineral specimen. Spessartite garnets can be red, reddish orange, orange, yellow-brown, reddish brown, or blackish brown. Malaya is a trade name for a pyrope-spessartite that varies in color from red, through shades of orange and brownish orange to peach and pink.
  • Andradite, Ca3Fe2(SiO4)3, Calcium iron silicate. Andradite forms in contact or regional metamorphic enviroments as does grossular, the calcium aluminum garnet. It is believed that these garnets form from the metamorphism of impure siliceous limestones. Andradite garnet can be yellow-green, green, greenish brown, orangy yellow, brown, grayish black or black. Melanite is a black titanium bearing variety of andradite. Demantoid is a rich green variety and Topazolite is the yellow variety and is also occassionally cut as a gem.
  • Uvarovite, Ca3Cr2(SiO4)3, Calcium Chromium Silicate is the only consistently green garnet and has a beautiful emerald-green color. As with the other calcium garnets (andradite and grossular), Uvarovite is formed from the metamorphism of impure siliceous limestones and some other rocks that contain chromium. Mineral specimens of Uvarovite are much sought after by collectors for outstanding brilliance and color.
  • A few garnets exhibit a color-change phenomenon and exhibit an "alexandrite-like" effect. They are one color when viewed in natural light and another color when viewed in incandescent light.

Legends, Myths and Healing Properties:

GarnetGarnet's powers include healing, strength, and protection. Legends dating from the Middle Ages suggest that primitive cultures thought red gems would stop bleeding. Garnets were believed to protect one from poisons and cure other sicknesses including those dealing with blood and infection and it is often worn to relieve inflammations of the skin. It is also believed to regulate the heart and blood flow and aid in curing depression. They were believed to have the powers to smooth discord between enemies and to prevent bloodshed. Garnet is considered the gem of faith and truth. In earlier times, garnets were exchanged as gifts between friends to demonstrate their affection for each other and to insure that they meet again. Garnet jewelry has been found that dates back to the Bronze Age (3000 BC). It is believed that garnet was buried within the tombs of the dead to offer protection through the death experience. Present day therapeutic beliefs include: Rhodonite, emotional support, helps one take the next step emotionally.