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Geology/question about a rock?

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my rock
my rock  
I found a rock while in Mesa Arizona! I think it may be a geode. I was wondering if you could tell me what kind it is! I have attached a picture as well!

Answer
The rock is a drusy botryoidal agate. Agate is the banded form of the mineral Chalcedony, which is a microcrystalline (very small crystaline) variety of Quartz. Agate is the most varied and popular type of Chalcedony, having many varieties on its own. Although the pattern on every Agate is unique, the locality of an Agate will provide resemblances in banding style and color, thus lending many Agates with a geographic prefix. Some examples are Laguna Agate (named after Ojo Laguna, Mexico) or Botswana Agate (after the African country of Botswana). Other variety names used connote specific colors or patterns, such as Fire Agate or Eye Agate.

Agate usually forms in rounded nodules or knobs which need to be sliced open to bring out the internal pattern hidden in the stone. Most Agate is ugly in its natural state; specimens must be polished to bring out their full beauty. Much of the Agate sold to collectors has been treated, in the form of tumbled stones or polished slabs. Popular collector forms of Agate include nodules or geodes sliced in the middle into two polished cross-sections, or thin slabs from nodule or geode cross-sections.

The formation of Agate is most often from deposition of layers of silica filling voids in volcanic vesicles or other cavities. The layers form in stages with some of new layers providing an alternating color. Since the cavities are irregularly and uniquely shaped, each Agate forms its own pattern based on the original cavity shape. When a cavity is completely filled, it forms a solid mass of Agate, but often it is only partially filled, leaving a hollow void which often has crystalline Quartz growths on its innermost layer. This is the cause of Agate forming the outer lining of most geodes.

Agate is often dyed to enhance its colors. This is especially true of Agate from Brazil. Bright neon colors such as bright blue and red are rarely natural.

Agate is named after the Achates River (now known as the Dirillo River) on the island of Sicily, Italy, whose upper waters were an ancient source of this gemstone. To see examples just search for "drusy botryoidal agate"  

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Joe Norris

Expertise

I am an economic geologist. An economic geologist does mineral evaluations and appraisals of mineral or mining properties. I can tell you if your deposit has value - remember that a mineral deposit, no matter how good, only has value when mined. Any value assigned to a mineral deposit, in the ground, is only the speculative value that deposit.

Experience

I have been a economic geologist for most of my 35 year career. Although I have done work in perhaps 45 states and numerious countries much of my work has been in Appalachian coal, intermountain west gold and silver, and Arizona uranium.

Organizations
Past President of the Virginia Section of the American Institute of Professional Geologists and a certified geologist in twelve states.

Education/Credentials
BS Degree from Eastern Kentucky University. Work on MS Degree @ Eastern Kentucky University, Colorad School of Mines & Marshall University Numerious short courses on the value of mineral deposits and how to value same. Also several short courses dealing with the different types of geologic processes; sedimentary, igneous, metamorphic along with the mineral associated with each.

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