Agate Terminology

(Last Modified November 27, 2011)




 
Agate Terminology

AGATE

Agates of Lake Superior by Dan and Bob Lynch is an excellent book that describes and shows pictures of many varieties of Lake Superior agates. They defined an agate as "The concentrically banded variety of chalcedony" which is stricter then I apply throughout my web page, and frankly their own book describes many varieties that defy the simple glossary definition. Most of the rocks on 123agates.com are agates, minus a few such as agate_316, for example. Agate_316 is a geode of Macrocrystalline quartz. The outer husk of agate_316 is chalcedony which, if their is enough of it, I tend to call "agate". A variety I call "Raisonettes" (I have hundreds), like agate_011 and agate_162 are botryoidal brown/red masses that do have chalcedony and resemble agates, but, are not. Although I do confess having slipped in a raisonette or two during our annual agate contests over the years!

ATTACHED NODULE

An agate that is still attached to its Basalt or Rhyolite host rock.

BANDED AGATE

An agate with lines.

BARITE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a soft mineral consisting of barium, sulfur and oxygen that forms blade-like crystals". 20110205_Blue_6 is a good example. In this example, the agate formed after, and around, the barite crystals.

BASALT

Dark gray or black lava rock. Agates form inside the vesicles. See also Rhyolite which is another common type of lava rock that agates form.

BOOB AGATE

Well, it kind of looks like a... ya know... boob. agate_079 is a fair example. A good boob is not as hard as it may seem to find.

CHALCEDONY

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "the microcrystalline variety of quartz composed of microscopic plate-like crystals arranged into parallel stacks". This is a dense form of quartz that is the essense of all agates. Throughout my web page you'll see reference to "hard whites". The white is Chalcedony and "hard" comes from the heavier touch and perceived good quality. agate_005, agate_013, agate_014 and agate_037 are good examples of what I call hard whites.

CHLORITE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a soft, dark green to black mineral that forms as a lining within vesicles". Jansen_2011_013 is a good example of it. Normally you don't see it because it weathers off easily.

CONCENTRIC BANDING

Exposed bands of nested rings that share a common center. jansen_003 is a good example. These typically have higher collector value.

CRATER

This is a rounded "indentation" sometimes found on the outside of nodules. agate_049 and Jansen_2011_009 are good examples. agate_052 may be evidence that a crater is actually an agate eye, or portion of, that popped out! A pity if so!

EYE AGATE

See the Paradise Beach Agate Eyes page for agate eye examples.

FRAGMENTED MEMBRANE

An agate where the outer vesicle membrane fractured into the vesicle and the agate formed around the parts. Basaltic is a great example, cut and photographed by Tom Shearer. This term is documented by Dan and Bob Lynch, authors of Agates of Lake Superior.

GEODE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a hollow rounded rock or mineral formation". agate_316 is a good example.

GOETHITE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a brownish yellow hydrous iron oxide". Some agates with yellow color may be caused by microscopic goethite impurities.

GRAVITATIONALLY BANDED AGATE

I pulled this term from Dan and Bob Lynch, authors of Agates of Lake Superior. This is a "water-level" agate where the bands are parallel to each other. The bottom of agate_040 is an excellent example.

HUSK

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "the thick, outermost chalcedony band of an agate that is composed of chalcedony spherulites". agate_012 is a good example of husk. agate_283 is an example of spherulites.

INFILTRATION CHANNEL

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a channel by which a jet of silica solution entered a vesicle". agate_153 is a good example.

LIMONITE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "the name given to yellow-brown unidentified hydrous iron oxides". What I found interesting in the book is that this agate color staining comes long after agate formation. Jansen_2011_007 is a good example. I call these "mustards".

LITTLE DOLLY

A small, perfectly shaped & weathered smooth agate about 1/8" to 1/4" big. Naturally polished! See field guide page 10 or group_003.

MACROCRYSTALLINE QUARTZ

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "Quartz crystals large enough to see with the naked eye". agate_316 is an excellent example.

NODULE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a round, compact mineral formation". Nodules is an entire page filled with classic Paradise Beach nodule examples.

PAINT AGATE

A creamy blend of tan, brown, orange and red color bands. Nodules is an entire page filled with classic Paradise Beach nodules, all of which are paint agates.

PEELED AGATE

An agate whose outer husk and/or inner banding has fractured away along band line(s). Jansen_2011_011 is a great example.

RHYOLITE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a light-colored quartz-rich rock formed when lava spilled onto the earth's surface". What I learned from the book is most of the agates on this web site formed either in Rhyolite or Basalt matrix.

SILICA

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "Silicon dioxide molecules, often in the form of quartz or dissolved in a solution." Silica solutions are essential for agate formation.

SPHERULITE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a very small spherical formation". Furthermore, inside the book the authors describe how these are concentrated on the outermost layer of husk. agate_283 is an example of two spherulites that were exposed after face polishing the agate.

STALACTITIC AGATE

Like stalactites hanging from a cave, but, in this case hanging from the ceiling of the vesicle and the agate formed around. Jansen_2011_005 is a great example.

STILBITE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a common zeolite mineral that primarily grows in fan-shaped crystal groups". agate_018 may be an example of this. Minerals in vesicles that come before agate formation may leave their imprint on the outer husk. When the nodule frees itself from the matrix during the course of erosion, the minerals long wear off, leaving only their fingerprint(s).

STRAWBERRY SHORTCAKE AGATE

I learned from Dan and Bob Lynch's book, Agates of Lake Superior, this is a Whorl agate. See group_008 and brilliant_011 for examples.

TRANSLUCENT

An agate that lets light through. agate_020 is a good example.

TUBE AGATE

An agate that formed tubes around mineral inclusions. The mineral inclusions may be lost and later replaced with Chalcedony. 20100101_tubes_stacked is a great example.

VESICLE

Dan and Bob Lynch, authors of Agates of Lake Superior, define this as "a cavity created in an igneous rock by a gas bubble trapped when the rock solidified".