prop agate   This article is about the semi-precious stone. For
other uses, see Agate (disambiguation).    The stone was given
its name by Theophrastus, a Greek philosopher and naturalist,
who discovered the stone along the shore line of the river
Achates sometime between the 4th and 3rd centuries BC.
Formation and characteristics  Most agates occur as nodules in
volcanic rocks or ancient lavas where they represent cavities
originally produced by the disengagement of volatiles in the
molten mass which were then filled, wholly or partially, by
siliceous matter deposited in regular layers upon the walls.
Such agates, when cut transversely, exhibit a succession of
parallel lines, often of extreme tenuity, giving a banded
appearance to the section. Such stones are known as banded
agate, riband agate and striped agate.  In the formation of an
ordinary agate, it is probable that waters containing silica in
solution -- derived, perhaps, from the decomposition of some of
the silicates in the lava itself -- percolate