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What are Agates, how do they form, and what patterns do they have?

Posted by Barlows Gems on 15th Jun 2019

What are Agates?

Agate is a member of the Chalcedony family.

Chalcedony (kal SED' nee) is any form of microcrystalline quartz, where the crystals are too small to be seen without high magnification.

However, in the rock world, only the translucent or transparent, single color types are sold as "chalcedony", such as Blue Chalcedony. Most of the other stones fall under the agate or jasper category.

Agates tend to have a more fibrous texture, where jaspers tend to be more granular.

Also Agate is usually defined by its translucency (ability to see light through it), and by its clearly defined pattern or banding of multiple colors. Jaspers are mainly opaque, with a solid color or an irregular display of colors caused by various mineral impurities.

Of course there are always exceptions!

How are Agates formed?

Most Agate was created by volcanic activity. The magma essentially had gas or air bubbles and over time silica rich solutions filled those pockets. That liquid form eventually solidifies. Agates may form in the shape of “thundereggs” or nodules, or may grow in cracks or lkayers, to form “vein” or "seam” agates.

Every Agate is unique. Though commonly banded, there are many variations of patterns in Agate stones caused by impurities present when the Agate formed

Inclusions occur when iron oxide, manganese oxide, or other oxides are present when the agate forms. These oxides are quite common. The inclusions grow and are supported in the agate when it is in a liquid or gelatinous state.

Many people collect agates specifically for their type of patterns or inclusions.

What type of inclusions do Agates have?

Tube Agate- a tube basically looks like a test tube, straw or worm. Depending on how the Tube Agate is cut you can see the long tubular shapes or you see concentric eye-type banding around the mineral inclusion. Most of the time the tube is filled, but sometimes the tubes do remain hollow.

Moss Agate- Moss inclusions look like growths of moss or hose-like tubes oriented in various direction. Though most people think of Moss Agate as white or milky clear with green inclusions, the moss inclusion can actually be several colors. The green coloring is caused by chrome or iron oxides.

Tree Agate- A Moss Agate that is opaque white with green tree-like inclusions.

Dendritic Agate- Dendrites are very thin and delicate, two-dimensional, tree like inclusions. Most often they are in clear or milky white agates and the dendrites are black, red or brown. The black is usually from manganese oxides and the red and brown come from iron oxides. The name Dendrite comes from the Greek word Dendron which means tree.

Plume Agate- Plumes are three-dimensional inclusions that resemble trees, feathers, or flowers. They are usually much thicker than the two-dimensional dendrite inclusions and thicker than Moss Agate inclusions.

Sagenite Agates- Sagenites are hair-like needles and often form in sprays. Most sagenite inclusions in agates are pseudomorphs, where there was an original mineral present and it was replaced by silica.

Metallic Inclusions- Chalcopyrite, pyrite, marcasite and copper inclusions can all be found in agates.

Eye Agates- Eyes are orbs that are concentrically banded giving the appearance of a bull’s eye. The shape of the eye is hemispherical, with the flat surface often oriented towards the exterior face of the agate.

Polka Dot Agate- Just like it sounds, the inclusions are spots or polka dots!

Pseudomorphs- A pseudomorph is when the original mineral is replaced by silica, but retains the characteristics of the original mineral. Petrified Wood is a good example of a pseudomorph because the wood was replaced by silica to form jasper that still looks like the original wood.

Iris Agates- Not really an inclusion, but more of a phenomenon of banding, Iris Agates or Rainbow Agates are finely banded white or colorless agates that when cut very thin and polished have a “reflection” an iridescence of bright, beautiful rainbow colors. This phenomenon occurs mostly in Mexico and Brazil agates.