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Sagenite

via The Rockhounder, July, 2012

Agates with inclusions are some of the rarest and most beautiful agates in the world. These inclusions may be sagenitic (sagenite), plume, dendritic, or moss. During our club's November field trip to the Afton Canyon area near Barstow, CA, we were directed to a barren hill in Baxter Wash. Sylvia Cliffe, one of our club's most experienced members, informed us that we were going to search the hillsides for Sagenite. This prompted the question: what is Sagenite? and your editor was tasked to provide an explanation. After some research I can report that the term “Sagenite” is perhaps a misnomer - a term that refers to a characteristic of agates whereby the material displays rays, or sprays, of crystal growth within the substance.

Collector Pat McMahon, of the Sedona, Arizona, Gem and Mineral Club, has identified sagenite from over 250 different agate deposits worldwide. She offers this information for our readers: Sagenite, or more accurately, sagenitic agate is any agate having acicular or needle-like mineral growths. These hair-like filaments are often arranged in fans or sunbursts and may come in a wide array of colors. My belief is that at least a little sagenite can be found at most agate fields. It is impossible to say today what percentage of the original deposits were sagenitic, but in the hundred or so agate fields I have been to, a very small percentage of the agate has sagenitic inclusions. With the exception of a small number of fields, probably less than five percent of the available agate at fields I've been to is sagenitic. Those few agate fields that have a higher incidence of sagenite offer a rare and exciting treat to the collector.

Plume is surprisingly more common than most of us might believe. Many collectors know of Priday Plume, Graveyard Point, Del Norte (Colorado), West Texas, and Mexican Plume. The oxides which form plume and other inclusions are quite common. If they are present and conditions are right, the inclusions form. I have plume from over a hundred agate fields in my collection. Inclusions occur where iron oxide, manganese oxide, or other oxides are present when the agate is formed. The oxide minerals grow in the agate when it is in a liquid or gelatinous state. The inclusions grow and are supported by this liquid medium. If sagenite inclusions grow outside of the gel, the tops of the needles resemble a pin cushion. Plume and moss inclusions grow outside the gel as well.

In very rare instances, plume and sagenite are found in the same rock. Plume and moss are often associated together. I have found sagenite in or near moss only a few times. Multicolor plumes appear to be more common than multicolor sagenitic sprays. I don't understand this. It may be coincidental. It is based on examining several thousand specimens. I have not yet seen plumes or sagenite needles penetrate bands in agate. My belief is that sagenite and plume are formed in silica gel after the gel fills or partially fills the cavity. Banded agate apparently forms at a different time. Some of my favorite agates have complete fortifications next to inclusions.

The thrill of finding a quality agate in the field is only topped by making that perfect, often lucky cut in the saw. Unlike banded agate which often gives the cutter multiple quality slabs, we seldom get more than one outstanding cut per rock with plume or sagenite. As a collector, I am very selective in what I pick up in the field and still less than 5% of what I cut goes in my display case. That's the nature of what we do. A very small quantity of agates are exceptional.