1. shebûw, derived from a root word meaning flashes or streamers. Exodus
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.
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
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
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
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
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.
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.
Darragh, P.J., A.J. Gaskin, & J.V. Sanders, 1976. Opals. Scientific American.
Hurlbut, op. cit.; 322.
Kunz, George Frederic, 1908. The curious lore of precious stones. New
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