Traditional Birthstones - Garnet (January)

Garnet

Garnet is a rather complex mineral that has a general chemical formula of R3R2(SiO4)3, where R3 is a bivalent (gives up two electrons) metal and R2 is a trivalent (gives up three electrons) metal when forming a chemical bond. The metal R3 may be Calcium (Ca++), Magnesium (Mg++), ferrous Iron (Fe++), or Manganese (Mn++), and the metal R2 may be Aluminum (Al+++), ferric iron (Fe+++), or Chromium (Cr+++). With so much substitution possible, the single garnet crystal may include a hodge-podge of elements, and garnets may be a multitude of colors; natural garnets are known in every color but blue.

Garnet crystallizes in the isometric system; the crystals have three axes that are all equal length and are perpendicular to oneanother. Garnet favors the dodecahedral (12 faces) crystal habit, but another common habit is a trapezohedron (24 faces). The dodecahedral crystal can be modified by the trapezohedron producing many smaller crystal faces. Hexoctahedral (48 faces) crystals are sometimes observed as are many other modifications of the basic isometric crystal plan. Garnet may also be found in massive forms and this was the foundation of a large abrasive industry in the United States. Much of the garnet abrasive has been replaced by manufactured silicon carbide but garnet paper is still available.

Because of the variation in chemical composition, bonds between some ions are stronger than those between other ions and the hardness may vary from about 6 ½ to 7 ½ on the MOHS scale, and the specific gravity (S. G.) (weight of the stone compared to the weight of an equal volume of water) may vary from about 3.5 to 4.3. The luster of garnets ranges from vitreous to resinous to subadamantine. Colors can be red, brown, yellow, orange, white, green, or black or shades in between.

Because garnet crystallizes in the isometric system, it is isotropic (has only one refractive index); that is, a beam of light passing through a garnet travels at the same velocity regardless of the direction of travel. The refractive index of a substance is defined as the velocity of light traveling through air divided by the velocity of light traveling through a substance and is given by the formula:

R. I. = Vair/Vsubstance

The refractive index of garnet can vary somewhat. These variations are helpful in determining the varieties of garnet but there can be some overlapping so other phenomena must be taken into account when working with garnet.

Pyrope, a magnesium-aluminum garnet is probably the most common gem variety. It is usually a dark red to black and is usually transparent only in fairly small stones. Much of the garnet of antiquity was probably pyrope and fine stones came from Czechoslovakia. Many tiny stones were drilled and the Czechs developed very fine beadwork with these. Its' R.I. ranges from about 1.72 to 1.75 and its' S.G. ranges from about 3.6 to 3.9 and is usually about 3.8. Pyrope may contain needle-like inclusions.

Almandite may be deep red or brownish red. Magnesium has replaced ferrous iron and ferric iron has replaced aluminum. The S.G. ranges from about 3.9 to 4.2 and is normally about 4.05. The refractive index is normally around 1.79 and varies only slightly above or below this parameter. Almandite may have enough silk (oriented needle like crystal inclusions) to produce asteriated (starred) stones of normally 4 rays. Other inclusions are zircon crystals that are frequently surrounded by dark haloes that are caused by damage from sub-atomic particles emitted by radioactivity in the zircon. Star garnets have been reported from only India and Idaho, the latter source being the more prolific. To produce the best stars, the lapidary should use one of the dodecahedral crystal faces as the base of the cabochon.

Rhodolite garnets are rose red to purple and in composition are about 2 parts pyrope to one part almandite. Rhodolites often contain snowflake like inclusions and some stones have numerous snow flake-like inclusions that produce the appearance of an "internal blizzard."

Spessartite is a manganese-aluminum garnet. It is usually brownish to red but some sources have yielded orange stones. It is not commonly seen in the gem trade and was almost unknown until the end of the 19th Century. Its' R.I. is about 1.81 and its' S.G. about 4.2. It may have wavy feather-like inclusions that are formed by minute liquid droplets.

Grossular garnet derives its' name from the color of the common gooseberry. A massive form of such color is often marketed under the name of Oregon Jade although the gem industry frowns upon such mislabeling. It is a calcium-aluminum garnet. Grossular garnets also come in shades of orange, red, and yellow. The term hessonite is applied to orange to yellow grossular; these stones have a syrupy or heat-wave effect and have rounded inclusions that resemble vitamin capsules. The S.G. of grossular ranges from about 3.58 to 3.73 and is normally around 3.61. Its' R.I. is in the range of 1.72 to 1.75, with the greens being in the lower range and the oranges in the higher range. Tsavolite (=Tsavorite) is an emerald green variety of grossular that has been known only since about 1970. It has been found only in Tanzania and derives its name from its source near Tsavo National Park.

Andradite is a calcium-iron garnet and it comes in shades of yellow, green, brown, and black. Its' S.G. is normally 3.84 to 3.85 and its' R.I. ranges from about 1.77 to 1.81, and is commonly about 1.79. Demantoid (diamond-like) is a Dutch term that is applied to transparent andradite that has a R.I near 1.81 and has a very high dispersion (the ability of a stone to break white light up into its component colors). A fine demantoid may cast the appearance of a yellow or light green diamond. Demantoids are usually characterized by horse-tail like inclusions of the amphibole mineral, byssolite.

Uvarovite is a calcium-chromium garnet that shares the emerald-green distinction of Tsavolite. Uvarovite is usually found only as very tiny drusy crystals and pieces large enough for faceting are virtually unknown. With the increase in popularity of jewelry using small plates of drusy crystals, uvarovite that was formerly confined to the mineral collector's cabinets has found its niche in the gem world.

Garnets may be confused with spinel which has lower physical and optical properties and corundum which is doubly refractive. Glass has bubbles and swirl lines that are easily seen under magnification.

References

  • Ford, W.G., 1958. A Textbook of Mineralogy with an Extended Treatise on Crystallography and Physical Mineralogy. John Wiley & Sons, Inc., New York, 851 p.
  • Gubelin, E.J., 1974. Internal World of Gemstones; Documents from Space and Time. Zurich, ABC Edition, 234 p.
  • Hurlbut, C., 1963. Dana's Manual of Mineralogy, 17th Ed., John Wiley & sons, New York, 609 p.
  • Rouse, J. D., 1986. Garnets. Butterworth's, London. 134 p.
  • Schumann, W., 1977. Gemstones of the World. NAG Press, Ipswich, 256 p.
  • Zeitner, J. C., 1996. Gem and Lapidary Materials. Geoscience Press, Inc., Tucson, Arizona, 347 p.