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 formation types.
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 parallel.
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 stone.
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 ferns.
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 silica gel.
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 themselves.
Bamboo Agate-
Plume agate-photo courtesy of Stones That Rock
