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Agate is a banded form of finely-grained, microcrystalline Quartz. The lovely color patterns and banding make this translucent gemstone very unique. Agates can have many distinctive styles and patterns, but each Agate is unique in its own habit, with no two Agates being the same. |
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Chemical Formula |
SiO2 |
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Color |
White, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black, Banded, Multicolored |
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Hardness |
7 |
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Crystal System |
Hexagonal |
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Refractive Index |
1.54 - 1.55 |
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SG |
2.63 - 2.65 |
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Transparency |
Translucent |
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Double Refraction |
.009 |
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Luster |
Vitreous |
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Cleavage |
Indiscernible |
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Mineral Class |
Quartz (Chalcedony). See also the Agate mineral page. |
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A generally accepted requirement in the definition of Agate is that it has to be banded.
This qualification distinguishes it from other forms of Chalcedony
which are not banded. Exceptions include Dendritic Agate and Moss Agate, which are not true Agates since they lack the banding patterns,
but they are still traditionally called Agates since they have more than one color. Onyx, when banded
white and black, is technically a form of Agate, and Sardonyx, which
is a banded reddish and white Chalcedony, is also technically a type of
Agate. Some forms of Carnelian may also exhibit banding and can
therefore be classified as both Carnelian and Agate.
Deposits where Agates are commercially mined are usually very extensive, thereby enabling this gemstone to be affordable and inexpensive. However, a fine and sharp banded pattern, combined with natural strong coloring, will quickly increase the cost and value of and Agate. Specific Agate localities will provide similarities in banding style and
color, thereby lending many Agates a geographic prefix. Other variety names used
will connote specific colors or patterns, such as Fire Agate or Eye Agate.
The history of Agate production is closely tied to German town of Idar-Oberstein, which has evolved as an important gemstone center. Agates and Jasper were historically found in the Idar-Oberstein region, and cut and polished by local craftsman. With the discovery of the enormous Agate deposits in Rio Grande do Sul, Brazil in the 1800's, the new material was shipped to Idar-Oberstein for cutting and polishing. The local Agate cutters of Idar-Oberstein were well-suited for this job, having been historically trained in this profession. Idar-Oberstain remains today as an important cutting and polishing center.
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Agate is generally an inexpensive gemstone when used in jewelry. It is cut and polished into cabochons, and used as beads for necklaces and bracelets. It is also carved into cameos which can be worn as pendants. Agate makes an exquisite ornamental stone, and is cut into slabs, animal carvings, ornate book ends, and small statues and figures. Ornaments such as knife handles, pins, snuff boxes and the like are also cut from Agate.
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Agate has an over-abundance of variety names. Some variety names are
generally used by
collectors and dealers, but there are many made up
by dealers to describe a locality or other habit. The
varieties below are the well-known names or varieties that are commonly encountered. Seldom-used and localized trade names are not described here. (For additional varieties, see the Agate mineral page varieties section.)
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Agate Geode
- Layer of Agate surrounding a cavity in a geode that is
usually lined with a layer of small Quartz crystals.
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Blue Lace Agate
- Agate with light blue bands in a lacy or wavy pattern.
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Botswana Agate
- Agate from the African country of Botswana banded with fine parallel lines of white, purple, or peach.
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Condor Agate
- Agate from San Rafael, Argentina, often with vivid colors.
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Crazy Lace Agate
- Agate with twisting and turning bands of various colors.
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Dendritic Agate
- Translucent Chalcedony with tree-like or fern-like inclusions.
Dendritic Agate is technically not a true Agate, as it lacks the
banding patterns exhibited in Agates.
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Eye Agate
- Agate with banded, concentric rings that are perfectly rounded.
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Fire Agate
- Form of Agate or Chalcedony that is
iridescent with a play of colors or "fire" similar to that of
Opal. Fire Agates usually have botryoidal growth included in their
interior. The play of color is caused by inclusions of Goethite or
Limonite.
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Fortification Agate
- Agate with a pattern in which all bands connect to each other,
causing it to resemble a medieval fortress (i.e. imaginary moat and
walls surrounding the castle).
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Iris Agate
- Rare iridescent Agate that exhibits spectral colors on a translucent colorless or white base.
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Laguna Agate
- Well known form of colorful Agate with very dense banding from Ojo Laguna, Chihuahua, Mexico.
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Moss Agate
- Chalcedony containing dense inclusions of green Hornblende
that cause the pattern to resemble moss. Moss Agate is technically not a true
Agate as it lacks the banding patterns of
Agate.
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Onyx
- Type of Chalcedony gemstone whose meaning can have several connotations. Its most practical gemstone definition describes a solid black Chalcedony, or a banded or layered black and white Chalcedony.
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Sagenite Agate
- Agate with acicular or or pointed inclusions of various
minerals. These hair like formations are often arranged in fans or
bursts.
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Sardonyx
- Form of Agate with parallel bands of brownish to red alternating with white or sometimes black bands.
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Snakeskin Agate
- Agate with a scale-like layer that resembles the skin of a snake.
Also refers to a reddish brown Agate with small black concentric bands.
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Agates used as gemstones may be naturally colored, but they are often dyed. This is especially true of the Agates from Brazil that have more intense colors. Neon-like colors of pink, blue, or green in Agate are always dyed.
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Agate deposits exist in Brazil, Argentina, Uruguay, Mexico, Poland, Botswana, India, Australia, and the United States (Oregon, Arizona, Montana, Wyoming, South Dakota, and Michigan).
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The banding habit of Agate can distinguish it from most gemstones. Purple Fluorite known as Blue John may be banded, but it is much softer. The banded form of Calcite/Aragonite known as Travertine may also occur multicolored and be similar to Agate, but it too is much softer.
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Most Agates are rough and dull in nature, with their banding patterns and color hidden until they are sliced and polished. The images below show examples of both Agates in their native, unpolished state, polished Agates, and combinations of the two.
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