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Tuesday, January 4, 2011

Garnet, January Birthstone

’Garnet’ is derived from the Latin word ‘granum’ or ‘granatus’ meaning seed, because small red garnet stones look like pomegranate seeds.  There are 15 species of garnet; almandine, an iron rich garnet, is the most common. Other common species are spessartine, hessionite, and grossular; grossular garnets are so named for their gooseberry green color.
Garnets can be found as inclusions in diamonds. Garnet is a naturally occurring impurity in marble and lends a brown color to the marble.
Cape rubies, Australian rubies, and Arizona rubies are all actually garnets.

Unusual quality.
Garnet produces star stones, a type of gemstone that exhibits chatoyancy, the reflection of light from tiny inclusions of impurities, such as titanium oxide. These impurities are forced to align during the cooling stage of the formation of the garnets.  Star stones have to be cut with a flat underside and domed top (en cabochon), with the dome oriented in a certain way relative to the impurities or no star can be seen. Garnets produce four, six, or eight rayed stars. Other stones that form stars are sapphire, ruby, quartz, and kornerupine. Garnet is the only stone that forms 8 rayed stars.

Garnet is January’s birthstone, and the stone for the zodiac sign Aquarius (Jan 21 – Feb 19); it is the suggested gemstone for a second wedding anniversary. Garnet is associated with the root chakra.
Many meanings for garnets can be found. Garnets are typically associated with guidance, security, and protection, true love, devotion, and constancy, and health, friendship, and good luck in business ventures.
Wine-red garnet is the state gemstone of New York; star garnet is the state gemstone of Idaho. Garnet is Connecticut's state mineral.

Properties and characteristics of garnets.
Hardness: 6.5 – 7.5 on Moh's scale; garnetsd cannot be scratched with a knife, but they can scratch glass (but cannot cut glass.) 
Luster: Vitreous, which means it has a shine resembling that of glass.
Transparency: Transparent to translucent.       
Group: Silicates. Other silicates include quartz, amethyst, agate, opal, labradorite, lapis lazuli, prehnite, tourmaline, emerald, peridot.  
Crystal system: Cubic. Dodecahedral and trapezohedral
Composition: Follows the formula A3B2(SiO4)5, where A can be calcium, ferrous iron, magnesium, or manganese, and B can be aluminum, ferric iron, chromium, manganese, silicon, titanium, zirconium, or vanadium. The identities of A or B are factors in the color of the garnet.
Color: 15 species of garnet, which range in color: dark red, red-violet, pink, reddish orange, cinnamon, honey, yellow, yellow green, emerald green, grayish green, black, brown, cream, white, and colorless.
Some localities garnets are found.
Quebec, Canada. Czech Republic. Brazil. Finland. Bavaria, Germany. Italy. Kenya. Mexico. Norway. Poland. Russia. Scotland. South Africa. Sri Lanka. Tanzania. Turkey. Arizona, Colorado, Georgia, Idaho, New Hampshire, New Mexico, North Carolina, Pennsylvania, Utah, and Virginia, USA.

Formation of Garnets.
To understand how garnets form, we need a quick review of middle school science with some general, brief definitions.

Rocks are made of minerals, and some minerals are gemstones.
Rocks are naturally occurring and coherent aggregate of one or more minerals.
Minerals each have a specific chemical composition and distinctive internal crystal structure.
Gemstones are minerals that are prized for beauty, durability, and rarity. 

There are three main classes of rocks: igneous, sedimentary, and metamorphic.
Igneous rocks are formed from molten rocks. Examples are granite, basalt, obsidian, and pumice.
Sedimentary rocks are formed by the accumulation of minute grains or by the precipitation of dissolved materials. Examples are limestone, chalk, rock salt, shale, and sandstone.
Metamorphic rocks are made when existing rocks are subjected to extreme temperatures and/or pressures different from the conditions under which the existing rock formed. Examples of metamorphic rocks are marble and slate.

Metamorphic rock formation.
An existing rock – either an igneous, sedimentary, or metamorphic rock – transmutes under the new temperature or pressure, the atoms and molecules actually rearranging into new materials while in a solid state, without melting! We are not talking about turning lead into gold; we are talking about a rearrangement of minerals.
Quick examples:
~Under thermal pressure, aluminum-bearing minerals such as feldspar -sandstones are changed to micas and garnets.
~An iron-rich garnet and a magnesian-biotite (a form of mica) under low-grade temperature or pressure changes can reassemble into magnesium-rich garnet and iron-biotite.
~Under more intense conditions, the garnet and the biotite may disappear, their components recombining to form other minerals or liquid.

Bonewitz, Ronald Louis. (2005) Rock and Gem. Dorling Kindersley Limited. New York.

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