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Tuesday, August 21, 2012
Gemstone Highlights: Agate
What is it?
Agate is a microcrystalline variety of silica, namely chalcedony. It is known by its fine grain and bright colors. Usually associated with volcanic rocks, it can also be common in metamorphic rocks. It was given its name by a Greek philosopher, Theophrastus, who discovered it on the river Achates (now called the Dirillo in Sicily) about the 3rd century.
Many agates are hollow. The last deposit commonly consists of quartz, often amethyst, in which the apices of the crystals are directed toward the free space. With this crystal-lined cavity, it is referred to as a 'geode'. In the blue or gray agates, clear quartz cystals, or "drusy" can be found, and is usually treated separately.
In the ancient Persian culture, items created with these gemstones were considered talismans, as the gem was associated with preventing storms and increasing crops. Agates were also popular with European royalty in the Renaissance as well as with the rulers of the Byzantine Empire. Agate has also been discovered in many ancient sites as one of the most commonly used materials in hard carved art and practical items including paper knives, inkstands and seals (picture on right). Due to its hardness (Mohs Scale 7) and acid resistance, it has been used for making mortars and pestles to crush and mix chemicals.
Common colors include yellow, black, purple, green, red, brown, orange, white, blue and red-brown, with green and blue agates being the rarest. When it is cut transversely, it exhibits a succession of parallel lines, giving it a banded appearance, which is also known as riband agate or striped agate. Although sporting a waxy luster, agate polishes well.
Where is it found?
Agates can be found all over the earth. However, the more important mines are found in the United States, Europe, Canada, and Russia.
Types of Agate.
There are thousands of named agates based upon where found, who discovered them, composition, color and other characteristics. There is a classification of nine types of agates that are based upon how they form which I found interesting in helping to gain a basic understanding.
Fortification Agates are the most common and are formed when bands crystallize into layers that follow the shape of the cavity. The banding is rigid-looking, and if viewing it from above, resembles a fort.
Water-line Agates are formed by silica rich water entering the cavity slowly with the excess water draining away creating separate layers forming a stack of parallel bands.
Shadow Agates are formed when the alternating translucent and opaque bands exhibit a shadow appearance when the agate is rocked back and forth (or by your orientation of view), and are considered an interesting optical effect.
Tube Agates begin when tiny hair-thin mineral rods grow first within the silica gel along which micro-crystals form. Sometimes the inclusions remain and other times they are hollow tubes.
Eye Agates have a mysterious appearance believed to be formed when the silica drains from the cavity leaving only a droplet to bead up on the cavity wall. The droplet then crystalizes into a solid chalcedony "eye."
Plume Agates have outside layers that form prior to the chalcedony bands. They are a filament growth of mineral inclusions that look like ferns or feathers. They are sometimes referred to as dendritic agates.
Geode Agates form when silica-rich water stops filling the cavity leaving a hollow center with a crystalline fill.
Moss Agates have clusters of mineral inclusions that resemble plants, trees or landscapes. Most inclusions are comprised of iron or maganese oxide.
Seam Agates form in the cracks of host rocks rather than rounder pockets. The deposits form parallel bands that fill the crack or seam.
Tips for keeping your agates beautiful and scratch-free include putting it on after you've applied make-up, hairspray or perfume. The chemicals in these cosmetics can damage the shine and create a build-up. Agate jewelry should be cleaned after wearing with a soft brush and cotton cloth, and each piece stored separately in a cotton cloth to avoid scratching with other agates or gemstones.
For thousands of years people have believed agates possess metaphysical properties. In researching the previous gems I have highlighted on this blog, agates by far have the most mystical properties attributed to them. Similar to other gems, the more common attributed properties of agates include protection, harmony, ability to calm, divert storms, stimulate fertility and personal creativeness. Some of the odder attributes include relieving thirst, helping one be truthful, keeping blood & bone marrow healthy, making one less disagreeable and more persuasive, and relieving allergies. Historically, agate was placed in water for cooking and drinking to dispel sickness.
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