Types of agate

A Mexican agate, showing only a single eye, has received the name of cyclops agate.
Included matter of a green, golden, red, black or other color or
combinations embedded in the chalcedony and disposed in filaments and
other forms suggestive of vegetable growth, gives rise to dendritic or moss agate. Dendritic
agates have fern like patterns in them formed due to the presence of
manganese and iron oxides. Other types of included matter deposited
during agate-building include sagenitic growths (radial mineral
crystals) and chunks of entrapped detritus (such as sand, ash, or mud).
Occasionally agate fills a void left by decomposed vegetative material
such as a tree limb or root and is called limb cast agate due to its
appearance.

Turritella agate is formed from silicified fossil Turritella shells. Turritella are spiral marine gastropods having elongated, spiral shells composed of many whorls. Similarly, coral, petrified wood and other organic remains or porous rocks can also become agatized. Agatized coral is often referred to as Petoskey stone or agate.

Greek agate is a name given to pale white to tan colored agate found
in Sicily back to 400 B.C. The Greeks used it for making jewelry and
beads. Today any agate of this color from Sicily, once an ancient Greek
colony, is called Greek agate. Yet the stone had been around centuries
before that and was known to both the Sumerians and the Egyptians, who
used the gem for decoration and religious ceremony.

Another type of agate is Brazilian agate, which is found as sizable
geodes of layered nodules. These occur in brownish tones interlayered
with white and gray. Quartz forms within these nodules, creating a
striking specimen when cut opposite the layered growth axis. It is
often dyed in various colors for ornamental purposes.

Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum due to the extreme delicacy of the successive bands, whence they are termed rainbow agates.
Often agate coexists with layers or masses of opal, jasper or
crystalline quartz due to ambient variations during the formation
process.

Other forms of agate include carnelian agate (usually exhibiting
reddish hues), Botswana agate, Ellensburg blue agate, blue lace agate,
plume agates, tube agate (with visible flow channels), fortification
agate (which exhibit little or no layered structure), fire agate (which
seems to glow internally like an opal) and Mexican crazy-lace agate
(which exhibits an often brightly colored, complex banded pattern) also
called Rodeo Agate and Rosetta Stone depending on who owned the mine at
the time.