Agate is a variety of chalcedony, a cryptocrystalline form of quartz. Translucency, patterns of color, or moss-like inclusions may distinguish this stone from other forms of chalcedony. Agates can show a wide variety of vivid, multiple colors. These are principally the result of traces of oxides of iron, manganese, titanium, chromium, nickel, and other elements. All agates take a wonderful polish and are tough enough for most jewelry uses. Designers often take advantage of the intriguing patterns these stones have to offer to create unique and fascinating pieces.
In general, agate values are quite modest. Their prices reflect mainly labor and artistry rather than the value of the material itself. Agates of large size or with particularly distinctive, fine, or landscape-like color patterns are at a premium. Custom cut pieces or stones from collectible locations would be substantially more expensive.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Chalcedony|
|Varieties||Fire Agate, Iris Agate|
|Refractive Index||1.530 to 1.543|
|Polish Luster||Greasy to vitreous|
|Fracture Luster||Dull to waxy|
|Hardness||6.5 to 7|
|Specific Gravity||2.60 to 2.64|
|Birefringence||Up to 0.004|
|Luminescence||Fluorescence varies with bands. Somewhat strong: yellow, blue-white|
|Enhancements||Dyeing (common, stable).|
|Transparency||Usually translucent. Occasionally transparent or opaque.|
|Absorption Spectrum||Dyed green gems show weak lines at 6450 and 6700 nm. Others not diagnostic.|
|Etymology||From the Ancient Greek Achates, the name of a river in southwestern Sicily where the material was found.|
Varieties of Agate
The color patterns in agates usually take the form of flat or concentric layers or bands. Mossy or dendritic inclusions can sometimes create the impression of vegetation and landscapes. Varieties of this gemstone are described by their color patterns, inclusions, or source.
With regular color layers and bright colors, this variety is one of the most popular. These stones are found all around the world, but Brazil is one of the most productive sources. Many of the richly colored, banded agates you see for sale are dyed.
Moss, Plume, and Dendritic
These stones contain mossy inclusions of mineral oxides that may be any color. Stones that present plant-like patterns are called moss agates. Those with feather-like patterns are called plume agates. Those with tree-like, branching patterns are called dendritic agates.
Picture or Scenic
Some inclusions can create the appearance of whole landscapes, complete with lakes, shorelines, trees, and shrubs, within an agate. Stones that show these “natural pictures” are treasured.
Inclusions of plate-like crystals of limonite make fire agates iridescent. Cutting and polishing can enhance this effect.
Noted for their delicate designs, these stones are banded with intricate swirls and loops. Mexico is one of the premier sources of this material.
These stones show iridescent colors reflecting from between the color layers.
Embedded, silicified shells create patterns in these agates. Turritella agate is composed mostly of shells and shell fragments of the gastropod turritella and certain other species.
Fossilized ancient tree trunks and limbs may have their organic components replaced by agate stone over millions of years. In some cases, their woody structure may also be preserved and visible with a microscope. The agate’s color may be very bright and strong.
A celebrated secret process for dyeing agates was developed in Idar-Oberstein, Germany, in the 19th century. When the agate deposits in this town were depleted, emigrants to South America shipped grey agates back to Idar-Oberstein. These pieces were dyed and the results were extraordinary.
Dyeing enhancements should be disclosed to consumers.
- Moss agates: India; Scotland; United States Northwest.
- Scenic agates: Yellowstone National Park; Wyoming; Montana.
- Banded agates: Brazil; Uruguay; Madagascar; Mexico; United States.
- Lace agate: Mexico; Arizona; Namibia.
- Fire agates: Mexico.
- Petrified wood: Arizona; New Mexico; California; Washington; Oregon; various European countries; many other localities.
Agates are usually nodular, but masses can be several pounds and many inches in diameter.