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Agate

Semiprecious Silica Mineral

AgateAgate is a common semiprecious silica mineral that has long been used as a gem and ornamental. Agate is the most common variety of chalcedony which is a form of quartz. Agate is, in fact, identical with quartz in composition and physical properties. Composed of silicon dioxide (SiO2), it has a hardness of 7, a glassy luster, a conchoidal fracture, and a specific gravity of 2.60.

Most agates occur in cavities of eruptive rocks or ancient lavas. These agates have a banded structure, successive layers being approximately parallel to the sides of the cavity. During crystallization, the colored bands are not disturbed. Varieties of agate are characterized by peculiarities in the shape and color of the bands, which are seen in sections cut at right angles to the layers.

Agate forms when gas bubbles trapped in solidifying lava become filled with alkali and silica-bearing waters, which coagulate into a gel. The alkali attacks the iron in the surrounding lava, and bands of the resulting iron hydroxide are created in the gel, which loses water and crystallizes, leaving the bands intact. Many agates, when cut in cross-section, reveal striking forms.

In riband agate, the bands appear as straight lines in cross section. Such agate, with white bands alternating with bands of black, brown, or red, is called onyx . A ring or eye agate has concentric circular bands of different colors. A variety having included matter of a green color, embedded in the agate and disposed in filaments and other forms suggestive of vegetable growth, is known as moss agate.

agate bookends

Agate Bookends made from cut and polished Agate. These are available in our online store, click on the pictures to see more bookends

Agate is found throughout the world. In the United States it is produced in several western states: Oregon, Washington, Idaho and Montana are the chief sources of these gemstones.

According to ancient superstition, wearing agate made one agreeable, persuasive, prudent yet bold. It brought God's favor and bestowed the power to vanquish enemies and acquire riches. White agate supposedly cured insomnia.

Chemistry
Crystal System
Hardness
 SiO2  Tetrahedral  5.5 - 6.5
Cleavage
 Fracture
Specific Gravity
 None Variable 2.6 (app.)
Color, Transparency
& Lustre
 VARIETIES 
>Variable
  • Eye
  • Moss
  • Onyx
  • Riband
  • Ring
  • White

Arizona is well known for its petrified wood agate because of the Petrified Forest National Park, and petrified wood ranks third in value of commercially produced gemstones. It is generally accepted that the Park contains the most colorful examples of silicified logs in the world.

Petrified wood occurs in every county in the State, but the commercial production is essentially from privately owned lands in Navajo and Apache Counties near the Petrified Forest.

agate in petrified wood
Petrified Wood

Arizona petrified wood has the broadest range of applications of any gem material produced in the State. The material is suitable for tumble polishing for use in baroque jewelry or for cutting into cabochons for jewelry and display. Freeform and calibrated slabs are polished for pen and pencil set bases and bases of other items, and polished slabs are used for clock faces. Additionally, large blocks, limb sections, and geometric shapes are used as bookends and decorator pieces. Objects of art, principally carvings, are produced, and furniture such as coffee and end tables are made from the petrified wood.

Arizona is the only State currently to have commercial production of fire agate. Fire agate is a form of chalcedony which contains inclusion of iron oxides that result in a play of colors much like that of precious opal. Eleven operations in Arizona report either commercial production of fire agate or dig-for-fee production. The material is produced in Graham, Greenlee, Maricopa, Mohave, and Yuma Counties.

Fire agate is cut into freeform and calibrated cabochons for use in the manufacture of silver and gold jewelry. The material is popular in the southwest and with hobbyist lapidaries throughout the United States. Fire agate also has been used and is currently used in Indian style jewelry.

California's "Mojave Blue" agate has gained a great deal of attention in the past several years. This pastel blue or blue-gray agate cuts into attractive cabochons for jewelry and, in the hands of an expert carver, makes outstanding carvings.

Colorado and Utah have deposits of fine quality jasper, agate, petrified wood, and agatized dinosaur bone. These deposits are found over a large area of both States and on both sides of the continental divide in Colorado.

 


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