Agates are some of the most creatively striped gemstones found in nature. Agates are found in all colors of the rainbow, with blue and green being the most unusual. Agates are a form of chalcedony (kal-sed'-nee) quartz, they are classically associated with volcanic rock. They differ from jasper in the fact that agates are composed of fibers of crystalline quartz, while jaspers are composed of grains of crystalline quartz. (Jasper will be the subject of another blog post.) Agate hardness on the Moh scale varies between 7 and 9.
Agates are formed when gas bubbles that are trapped in solidifying
lava become filled with alkali and silica-bearing waters and other
mineral impurities, which then gel. The alkali attracts the iron in
lava, and the bands of iron hydroxide are created in the gel. Over
time, the gel loses water and crystallizes, leaving the bands intact.
This is why agate is often found with rings like a tree trunk. It can
have tiny quartz crystals, called drusy or druzy, form within that
will add sparkle and uniqueness to the stone. Drusy is sometimes cut
from the stone and used by itself.
Agates are found all over the world, including Africa, Asia,
Brazil, India, Italy, Mexico and the USA. Agates are usually named
for the place they are found, but there are different types of banded
formations that are added to their name. Listed below are some of the
Fortification agate- in this type of agate, bands crystallized
into concentric layers that basically followed the shape of the
cavity it formed in. When sliced, the bands resemble the aerial view
of a fort.
Water line agate or onyx- in this agate formation, gravity
controlled the formation of the bands. The solution either entered
the cavity slowly, allowing for one band to form at a time, or it
drained out slowly. The bands in these agates are almost perfectly
Shadow agate- these agates are formed with alternating translucent
and opaque bands. The shadow effect occurs when light moving across
the surface causes what appears to be movement across the face of the
Eye agate-this unusual characteristic forms when most of the
silica drains from the cavity, leaving behind only droplets that bead
up on the inner wall of the cavity. Sometimes crystal growth will
continue from this drop similar to the formation of stalactites in
caves. Later, the cavity fills in with the usual deposits of
chalcedony micro crystals.
Plume agate- these agates first had formations on the outside
layer that formed before the deposit of the chalcedony bands. They
are filament growths of mineral inclusions that look like feathers or
Moss agate-this type of agate has mineral inclusions that look
like landscapes, plants or trees. These inclusions usually consist of
iron or manganese oxide. Sometimes the inclusions inhibit the
chalcedony banding and let the mineral clusters freely grow in the
Seam agate- these agates grow in cracks in the host rock, rather
than in cavities. The bands form in parallel rows that follow and
fill in the crack or the seam.
Somewhat in a class by themselves are Lake Superior agates. These
agates are only found in a certain region of the US. Glacial activity
spread the agates throughout Minnesota, the Upper Peninsula of
Michigan, and extreme Northwestern Wisconsin. They can also be found
around the Thunder Bay in
NW Ontario, Canada. Lake Superior agates are known for the rich
orange, red and yellow coloring. This comes from the iron that is
leached from rocks formed millions of years ago from iron rich lava.
Agates are said to be very powerful gemstones. The healing and
metaphysical properties are as varied as the colors of the agates
Plume agate-photo courtesy of Stones That Rock