Region III Quarterly

Volume 3 - No. 1

January, 1941


By Howard R. Stagner,
Park Naturalist,
Petrified Forest National Monument.

There is enough agate in the Petrified Forest National Monument, Arizona, to provide a pound of semi-precious stones for each man, women, and child in the United States. These stones - chalcedony, carnelian, jasper, onyx, etc. - have an estimated minimum value of $5,000,000, based upon a "timber cruise" over part of the area that accounted for 3,500,000 board feet of petrified wood. A survey of the entire forest would probably double this figure, without even considering the large amounts that are believed to be underground. The average valuation is about ten cents per pound.

The minerals in the petrified wood, so diverse in color and form, are but different variations of one chemical material. We give the varieties many names, but essentially all are forms of silica, which is a common constituent of the earth's crustal rocks. It makes up about half of the composition of these rocks. In granite and in sandstone we recognise silica in the form of small, glassy quartz grains, but in most other rocks the silica, although present in quantity, is locked up, its identity hidden, as it exists in combination with alumina, potash, iron, water and other rock-forming materials. It is the left-over silica, the surplus after Nature has used the needed amount in building up the minerals, or the silica salvaged when these minerals are torn down by weathering, that is deposited to form chalcedony, agate, and quartz.

Quartz is silica in crystalline form, or of crystalline internal structure. Chalcedony is amorphous or non-crystalline silica. There are other types but these are the common, general classes, the two that are of chief interest in the Petrified Forest. All of the semi-precious stones associated with the petrified wood are varieties of those two forms of silica.

Moss agate

Now, let us take a look at a polished cross-section of a petrified log, and see just what those varieties are. Here we see several large, irregular spots of white or translucent blue-gray. This is chalcedony in its natural color, unstained by impurities - pure, amorphous silica. The rest of the log consists of the same basic material, but is stained a variety of shades and colors. Over here the chalcedony is delicately tinted pink or red, and we call this color phase carnelian. The red deepens over here into the flaming opaque color of jasper. This side of the log is predominantly yellow, grading from a pastel shade to brilliant canary yellow, and over here is a spot of a rare, dark green. Thus we add yellow jasper and green jasper or plasma to our list of stones. Black areas stand out here and there in sharp contrast to the white and colored wood - jet black, glistening onyx. Near the circumference of the log we see what appear to be miniature landscapes, traced in the white wood in black lines and filaments. This is moss agate.

The entire log, then, is composed of silica, but of a variety of colors arranged in a mottled pattern. We call individual fragments of the log jasper, carnelian, onyx, or chalcedony, depending upon the color; but one name - agate - describes the mottled color of all. Agate is silica of a varied color, with a wavy band or mottled pattern. It is colored silica suitable for polishing into ornaments and jewelry settings. In cracks and cavities in the log we find many varieties of quartz. The clear or white quartz is pure, crystalline silica; and all of the crystals, regardless of color, have the same six prism-like sides capped with a six-sided pyramid. The most valued of all the quartz crystals is amethyst.

In the early days before this area was established as a national monument, many commercial and amateur gem hunters visited the Petrified Forest in search of amethysts. Logs were blasted to obtain the crystals. Many of the stones were of gem quality, some as large as two or three inches in diameter, and the rich lavender color gave the stones a value far exceeding that of the petrified wood. Surprisingly, these deep lavender crystals have no more coloring material, no more pigment than the clear, colorless quartz. What then gives amethyst its rich color? Scientists are not entirely agreed, but the color is probably a physical one like the colors of the rainbow, and is the effect of some peculiar internal crystal structure on light passing through the quartz.


Some of the smoky quartz, like the amethyst, owes its color to a physical cause, and contains no black coloring material. Other black crystals, however, contain small particles of black iron oxide pigment, Occasionally, too, red and yellow crystals are found, and more rarely, green quartz; and in these the color is produced by iron pigments. Some of the red and yellow crystals have a very interesting construction. The pigment is not disseminated throughout the quartz, but occurs in one or more layers within the crystal - layers that follow the exact outer crystal shape of the quartz. It is as if a small quartz crystal were first formed, then covered with a thin layer of red or yellow iron oxide, and completed by the addition of more clear quartz. The yellow variety is sometimes called "false topaz", but citrine is a better name.

The petrifying process has stopped in northern Arizona. No more agatized wood is being produced, and when one fragment is removed, it is gone forever. Federal law provides penalties for removing even as much as a chip of this petrified wood from the Petrified Forest National Monument.

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Date: 17-Nov-2005