Field Museum of Natural History 
DEPARTMENT OF GEOLOGY 
Chicago, 1927 
Leaflet Number 8 
Agate— Physical Properties and Origin 
Agate is a variety of chalcedony chiefly distin- 
guished by its banded or variegated structure and col- 
oring. Its banded appearance is due to the fact that 
it is made up of a great number of exceedingly thin 
layers which appear as bands in cross section. As 
seen by the naked eye, these bands differ considerably 
in width, some seeming no wider than a line, while 
others may be a quarter to a half inch wide. In real- 
ity, all these bands are made up of still finer ones, the 
individuals of which can be seen only with a micro- 
scope. In a section of agate only one inch in thick- 
ness, Sir David Brewster counted 17,000 such individ- 
ual bands, and this number is probably representative 
of their abundance in most agates. In all agates the 
layers are composed of minute fibers or microscopic 
crystals standing at right angles to the course of the 
bands. Agate is generally semi-transparent rather 
than opaque but the layers may differ considerably in 
this respect and some be quite opaque. These differ- 
ences in translucency correspond to variations in por- 
osity, as is shown by the fact that whereas the trans- 
lucent layers absorb coloring matters readily, the 
opaque layers are little penetrated by them. As to the 
average size of these pores in agate, it is known that 
the diameter of the pores of water-rich silica colloids 
(jellies) is about one five-millionth of a millimeter, 
and those of agate (which is silica without water) 
[105] 
