12 
Field Museum of Natural History 
In this way the forms known as pipe agates are sup- 
posed to originate. 
Natural Colors of Agates 
While the majority of agates are colorless in their 
natural state, some natural colors do occur. Red and 
brown are the most common and are due to a content 
of iron oxide. If the color is brown, iron hydroxide 
is the coloring matter, if red, iron oxide. The red 
color is usually produced by alteration of the brown, 
loss of water by exposure to sun's heat forming the 
oxide from the hydroxide. In agates with straight 
bands these colors form the layers of sard and car- 
nelian so much used for cameos. Another color na- 
turally occurring is black. It rarely occurs massive, 
usually appearing as scattered flakes. Oxidation often 
causes this to change to red or brown, usually in spots 
or along clefts. Green is a color often seen in the moss 
agates and adds to the impression that these agates 
contain real moss. It is now well known, however, 
that the moss-like appearance is not due either in color 
or form to moss. As already noted, green, fibrous in- 
clusions can be produced in water-glass, the colloidal 
form of silica, by introducing iron vitriol. Thread- 
like and moss-like forms may then appear as shown 
in Plate V. Whether the green fibers of moss agates 
are of this nature or consist of some fibrous mineral 
such as hornblende or chlorite is not certain. Perhaps 
both modes of origin occur. 
Artificial Coloring of Agates 
Nearly all agates that one sees in use nowadays 
have been artificially colored. While the possibility of 
coloring agates artificially has been thought to have 
been known to the Romans, and as having been men- 
tioned by Pliny, it is shown by Dr. Laufer in the later 
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