Editing Agate (section)

==Varieties (by structure)==
Agates are broadly separated into two categories based the type of banding they exhibit.<ref name="lynch water-level" /><ref name="pabian" />{{rp|pp=24,36}} ''Wall banding'', also called ''concentric banding'' or ''adhesional banding'', occurs when agate bands follow the shape of the cavity they formed in. ''Level banding'', also called ''water-level banding,'' ''gravitational banding'', ''horizontal banding'', ''parallel banding'', or ''Uruguay-type banding'', occurs when agate bands form in straight, parallel lines. Level banding is less common and usually occurs together with wall banding.<ref name="mindat" />

=== Wall-banded agates ===
''Fortification agates'' have very tight, well-defined bands. They get their name from their appearance resembling the walls of a [[Fortification|fort]]. Fortification agates are one the most common varieties, and they are what most people think of when they hear the word "agate".<ref name="lynch fortification">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=22–25}}</ref>

''Lace agates'' exhibit a [[lace]]-like pattern of bands with many swirls, eyes, bends, and zigzags. Unlike most agates, they usually form in [[Vein (geology)|veins]] instead of [[Nodule (geology)|nodules]].<ref name="pabian" />{{rp|p=31}}

''Faulted agates'' occur when agate bands are broken and slightly shifted by rock movement and then re-cemented together by chalcedony. They have the appearance of rock layers with [[Fault (geology)|fault lines]] running through them. ''Brecciated agates'' have also had their bands broken apart and re-cemented with chalcedony, but they consist of disjointed band fragments at random angles.<ref name="lynch brecciated">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=70–73}}</ref><ref name="pabian" />{{rp|pp=24,28}} They are a form of [[breccia]], which is a textural term for any rock composed of angular fragments.<ref name="lynch brecciated" /><ref>{{Cite web |title=Brecciated agate |url=https://www.mindat.org/min-7593.html |access-date=February 11, 2025 |website=Mindat.org |publisher=Hudson Institute of Mineralogy}}</ref>

''Eye agates'' have one or more circular, concentric rings on their surface.<ref>{{Cite web |title=Eye Agate |url=https://www.mindat.org/min-7598.html |access-date=February 11, 2025 |website=Mindat.org |publisher=Hudson Institute of Minerology}}</ref> These "eyes" are actually [[Hemisphere (geometry)|hemispheres]] that form on the husk of the agate and extend inward like a bowl.<ref name="lynch eyes">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=82–85}}</ref> ''Tube agates'' contain tunnel-like structures that extend all the way through the agate.<ref name="lynch tubes">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=64–67}}</ref> These "tubes" may sometimes be banded or hollow, or both. Both tube and eye agates form when chalcedony grows around a needle-shaped crystal of another mineral embedded within the agate, forming [[Stalactite|stalactitic]] structures. Visible "eyes" can also appear on the surface of tube agates if a cut is made (or the agate is weathered) perpendicular to the stalactitic structure.<ref name="pabian" />{{rp|pp=27,35}}

''Dendritic agates'' have dark-colored, fern-like patterns ([[Dendrite (crystal)|dendrites]]) on the surface or the spaces between bands.<ref name="lynch dendritic">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=78–81}}</ref><ref name="pabian" />{{rp|p=25}} They are composed of manganese or iron oxides. ''[[Moss agate]]s'' exhibit a [[moss]]-like pattern and are usually green or brown in color. They form when dendritic structures on the surface of an agate are pushed inward with the silica gel during their formation. Moss agate was once believed to be petrified moss, until it was discovered the moss-like formations are actually composed of [[celadonite]], [[hornblende]], or a [[Chlorite group|chlorite]] mineral. ''Plume agates'' are a type of moss agate, but the dendritic "plumes" form tree-like structures within the agate. They are often bright red (from inclusions of [[hematite]]) or bright yellow (from inclusions of [[goethite]]).<ref name="pabian" />{{rp|pp=32–33}} While dendrites frequently occur in banded agates, moss and plume agates usually lack bands altogether. Therefore, they are not true agates according to the mineralogical definition.<ref name="mindat" /><ref>{{Cite web |last=Team |first=Geology In |title=Moss Agate: Formation, Occurrence, Uses |url=https://www.geologyin.com/2020/02/what-is-moss-agate.html#google_vignette |access-date=2025-02-11 |website=Geology In |language=en}}</ref>

''Iris agates'' have bands that are so microscopically fine that when thinly sliced, they cause white light to be [[Diffraction|diffracted]] into its [[spectrum|spectral]] colors. This "iris effect" usually occurs in colorless agates, but it can also occur in brightly colored ones.<ref name="pabian" />{{rp|p=30}}

''Sagenitic agates'', or ''sagenites'', have [[Acicular (crystal habit)|acicular]] (needle-shaped) inclusions of another mineral, usually [[anhydrite]], [[aragonite]], [[goethite]], [[rutile]], or a [[zeolite]]. Chalcedony often forms tubes around these crystals and may eventually replace the original mineral, resulting in a [[pseudomorph]].<ref name="pabian" />{{rp|p=34}} The term "sagenite" was originally a name for a type of rutile, and later [[rutilated quartz]]. It has since been used to describe any quartz variety with acicular inclusions of any mineral.<ref>{{cite web |title=Sagenite |url=https://www.mindat.org/min-8578.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref>

=== Level-banded agates ===
Agates with level banding are traditionally called ''[[onyx]]'', although the formal definition of the term onyx refers to color pattern, not the shape of the bands.<ref>{{cite web |title=Onyx |url=https://www.mindat.org/min-2999.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=February 9, 2025}}</ref> Accordingly, the name "onyx" is also used for wall-banded agates. Onyx is also frequently misused as a name for banded [[calcite]]. The name originates from the Greek word for the human nail, which has parallel ridges.<ref name="pabian">{{Cite book |last=Pabian |first=Roger |title=Agates: Treasures of the Earth |last2=Jackson |first2=Brian |last3=Tandy |first3=Peter |last4=Cromartie |first4=John |date=2016 |publisher=Firefly Books |isbn=978-1-77085-644-8}}</ref>{{rp|p=37}} Typically, onyx bands alternate between black and white or other light and dark colors. ''Sardonyx'' is a variety with red-to-brown bands alternated with either white or black bands.<ref>{{cite web |title=Sardonyx |url=https://www.mindat.org/min-7604.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date= February 10, 2025}}</ref>

[[Thunderegg|Thunder eggs]] are frequently level-banded, however they may also have wall banding. Level banding is also common in [[Lake Superior agate]]s.<ref name="lynch water-level">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6 |pages=34–37}}</ref>

<gallery mode="packed" heights="130px">
Agate 6 (32375570980).jpg|Agate with both wall banding (top) and level banding (bottom)
Agate Braziilia.jpg|Brazilian agate with classic fortification banding
Crazy Lace Agate 06.jpg|Crazy lace agate
Eyeballed by all the eye agates (27395607964).jpg|Tumbled Lake Superior eye agates
Detail, Dendritic agate (cropped).jpg|Dendritic agate from India
Four moss agate cabochons.jpg|Moss agate cabochons
Iris Agate from (Agatized Petrified Wood), Stinking Water, Oregon detail, from- Oregon004 (cropped).jpg|Iris agate from petrified wood
Agate D Bruyere.jpg|Level-banded agate
Onyx Mainzer Becken.jpg|Onyx agate
Thunder Egg Agate (Priday Blue Bed, John Day Formation, Miocene; near Madras, Oregon, USA) 5.jpg|Level-banded thunder egg from Oregon, USA
</gallery>