THE GOLD OF THAT LAND: Biblical Minerals & Rocks  


 

2.     agate


Hebrew:
1. shebûw, derived from a root word meaning flashes or streamers. Exodus 28:19, 39:12
2. kakdôd, from a root word suggesting a deep fire or sparkle. Ezekiel 27:16; Isaiah 54:12.
Greek: achates, a beautiful stone from the river Achates in Sicily.


Probable Identification: agate or precious opal.


Mineralogy:
Agate in modern usage is a banded form of the mineral chalcedony, which is translucent, fibrous, cryptocrystalline quartz (SiO2). Agate displays alternating layers of white to bluish opal and brown cryptocrystalline quartz. The occurrence of agates as nodules gives its bands a concentric sub-spherical form. The bands are typically irregular and diffuse, with a low contrast in color and intensity. Agate feels slick, has a vitreous luster, and breaks with a conchoidal fracture.
Onyx and sardonyx are closely related forms of chalcedony. The banding of onyx shows a sharp contrast between parallel black and milky-white layers. In sardonyx, the contrast is between regular white chalcedony and red-brown to red layers of carnelian.
Moss agate takes its name from moss-like patterns of dark manganese oxide inclusions with a dendritic habit against a light-colored chalcedony background. Color and patterns of dendrites sometimes give the effect of wooded scenery in scenic agate. Other inclusion patterns resemble feathers in feather agate. Polka dot agate has dark, round inclusions in a lighter background.
Agates in India, Brazil, Mexico formed as geodes in "vesicles" or cavities in basalt and volcanic tuffs. In the Lake Superior region of the United States and Canada, agates and a variety of collectible minerals filled vesicles in ancient basalt flows. Weathering exposes them at the surface as resistant gravels, nodules, and boulders. Geodes are globular, hollow bodies ranging in diameter from an inch (2.5 cm) to over 3 ft (1 m), that contain roughly concentric layers of crystals precipitated from solution in groundwater. In volcanic and igneous rocks, the layers begin with microcrystalline chalcedony and become gradually paler and coarser toward the center. Geodes in limestones typically contain calcite crystals.
Opal falls on the borderline of the mineral kingdom because it is not quite crystalline and yet not completely amorphous or without crystalline structure. Scanning electron micrographs reveal that opal consists of a three-dimensional array of spheres of colloidal silica (SiO2.nH2O), 50 to 400 nanometers in diameter (1 nanometer is a billionth of a meter: 1 micron = 1 x 10-9m). Hydrous silica gel fills the voids between the spheres. Precious opal contains regularly stacked spheres of the same larger diameter (300 to 400 nm), making it a colloidal crystal, whereas common opal or potch displays a disorderly stacking of much smaller spheres (less than 250 nm diameter). Stacking and layering in precious opal produce opalescence, a rainbow-like play of colors, by dispersing and reflecting light in the same way that raindrops form a rainbow.
The percentage of water in the form of hydrous silica gel in opal varies, and precious opals with the greatest play of colors contain the highest percentage, from 6 to 10%. The same specimens consist of larger spheres with the most uniform size and packing. The body colors of precious opal range from white to black, depending on the presence of impurities. Black opal is considered the most valuable.
Opal's quasi-crystalline structure makes it softer than quartz, with a hardness between 5 and 6.5. Its specific gravity of 1.9 to 2.2 is also lower than quartz. Like other forms of quartz, opal breaks with a conchoidal fracture but has no cleavage.
Precious opal occurs in volcanic rocks or replacing carbonates in arid environments. The precious opal of southwest Queensland and New South Wales, Australia occurs as nobbies or nodules in deeply weathered soil profiles formed on the Winton Formation and other sediments of Cretaceous age. Intense weathering dissolved clay minerals and precipitated silica gel in voids within iron-rich concretions. Other deposits in New South Wales occur as cavity fillings in basalts and other volcanic rocks.
Australia supplies over 80% of our precious opal today. Honduras, Hungary, Mexico, and the United States also produce opal.


Historical Background:
Agates, carnelian, and lapis lazuli were the most popular gemstones in ancient Mesopotamia. Agate beads or pappardillû cut to resemble fish eyes became such an important trade item in the Persian Gulf region about 2000 BC that the Assyrians developed a method of counterfeiting them with artificially stained or heated chert and chalcedony. Pliny the Elder attributed the name agate to Sicily's River Achates, an early source of agates.
Although opal is common, precious opal is scarcer than diamonds. Hungarian opal was the main supply available to the ancient world, and it lacks the fire and color of Australian opal.

Sources:
Darragh, P.J., A.J. Gaskin, & J.V. Sanders, 1976. Opals. Scientific American. 232–4: 84-95.
Hurlbut, op. cit.; 322.
Kunz, George Frederic, 1908. The curious lore of precious stones. New York: Century.
Lucas & Harris, op. cit.; 386-387.
Quick, Lelande, 1963. The book of agates. New York: Chilton.
Ralph, Jolyon, 1993-2004. Http://www.mindat.org/search.php?minname=agate&submit2=search


 


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Last updated: 05/13/06.