UtahRockHound
spending too much on rocks
Sometimes your the Windshield, sometimes your the Rock.
Member since May 2008
Posts: 301
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Agate
Mar 12, 2009 23:17:15 GMT -5
Post by UtahRockHound on Mar 12, 2009 23:17:15 GMT -5
This may sound odd, but what is an agate. Different people have different ideas as to what makes an agate.
When I first started taking rockhounding serious, I seen several cases where someone you thought knowledgeable call a stone an agate.
Then others would call call chert or jasper with banding an agate simply because it has banding. This lead to some confusion on my part. Even printed in books, a stone was called an agate because it had banding.
In my opinion, an agate is silicon based quartz with definite banding, not a slow blend of colors. Bands can be nearly any color, but the distinction is definite edges to the banding.
Moss agate seems to be an excepted miss name. To me, it is silica based quartz, but it is not an agate. It is a blend, and not a band. So what should it be called.
Mostly I am curious what others opinion are, and thoughts on this.
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Agate
Mar 13, 2009 6:33:07 GMT -5
Post by Hard Rock Cafe on Mar 13, 2009 6:33:07 GMT -5
I don't think there is a clear cut way to define it, since it is a rock (contains multiple minerals), not a single mineral. Each of the identifiers below has exceptions.
The one hard and fast rule is that it must be made of cryptocrystalline/micro-crystalline quartz (chalcedony). Unfortunately, carnelian, chert, chrysoprase, heliotrope, jasper, and onyx are all chalcedony, which is one source of confusion.
Transparency is also key; at least some of the stone must be transparent. But, Lake Superior Agates (for example) often do not have transparent areas, but is there any doubt about what it is?
While banding is a clear identifier, it is not always required (IMHO). All banded rock is not agate, and not all agates are banded. For example, moss (which you mentioned) and plume are regarded as agate by most. Onyx seems to be called agate about half the time; maybe because there is no transparency? What about Turritella? Again, talking about the type of agates I'm most familiar with, Lakers, there is no doubt that eye agates were formed right along with the banded variety.
I'm sure others will chime in. Maybe Mel can enlighten us all.
Chuck
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Agate
Mar 13, 2009 6:43:28 GMT -5
Post by texaswoodie on Mar 13, 2009 6:43:28 GMT -5
Oh man, are you going to wish you hadn't asked that when Mel sees this!
The original definition of an agate was translucent and banded. Now if it looks pretty, it's an agate for some people.
Curt
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Sabre52
Cave Dweller
Me and my gal, Rosie
Member since August 2005
Posts: 20,472
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Agate
Mar 13, 2009 7:56:47 GMT -5
Post by Sabre52 on Mar 13, 2009 7:56:47 GMT -5
*L* Sorry but Mel ain't a real fast typist and we've had looooooong threads on the subject before.....Mel
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NorthShore-Rocks
fully equipped rock polisher
Member since December 2008
Posts: 1,004
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Agate
Mar 13, 2009 8:35:44 GMT -5
Post by NorthShore-Rocks on Mar 13, 2009 8:35:44 GMT -5
Here's a link to one of those discussions on this forum: forum.rocktumblinghobby.com/index.cgi?action=display&board=gloss&thread=26401&page=1Mel...you're not kidding (that's a lot of keystrokes!)This helped me and I thought this was a clear an excellent description. Mel wrote: OK, I'll give it a go *S* Chalcedony, flint, chert, agate etc are all microcrystalline quartz. Jasper is kind of more fibrous massive microcrystalline quartz. Now, here comes the confusion we get into with common names. Chert is a term sometimes used to describe poor quality jasper but in actuality, chert and flint come from marine sedimentary deposits whereas jasper comes from volcanics or metamorphosed volcanic sediments. True jasper is supposed to be opaque but then we get into the common named jasper like Stone Canyon etc which are part agate which we should call jasper/agate. Flint and chert can be opaque or translucent either entirely or in part. Agate used to be a term which applied only to true banded agate but now is used as common names for lots of included jasper/agates ie. Moss agate, plume agate etc. Chalcedony is usually thought of as clear to colored microcrystalline quartz with the norm being basically clear colorless to gray, red being carnelian, yellow being sard etc. Technically, agate should come from deposits associated with volcanics ie, Brazilians, Botswanas, Lakers etc. However, when we get into common names, Tee Pee Canyons, fairburns, Dryheads, Kentucky's etc are all called agate because of the banding when they are sedimentary in origin and should probably be classed as banded chert. So, I guess to summarize, and some may not agree:
All are microcrystalline or microfibrous quartz ( chalcedony) with various amounts of mineral inclusions which give the wide variety of color and pattern.
Agate ( common name) is banded and can be sedimentary or volcanic in origin.
Jasper and agate are basically volcanic in origin. Volcanic agate and jasper are often associated with ash, rhyolite and basalt formations.
Genesis of the silica in volcanic agate/jasper is overlaying ash deposits, Genesis of the silica in cherts and flints is thought to be organic sources like sponge spicules and radiolarian skeletons.
Flint and chert are sedimentary in origin ( often associated with limestone deposits) Oh yeah, and Some definitions classify flint as a fine quality translucent subdivision of chert with chert being more opaque but some chert is quite translucent and to my way of thinking much like flint.
Agate + jasper...volcanic
I'm sure I've messed up this explanation somewhere and lots of folks might disagree with my thinking.*L* as the common lapidary material names mess me up all the time. For instance, lots of jaspers are not jasper at all. Poppy jasper is more of a chert and lots of picture jasper is kind of a silicified volcanic mud much like rhyolite in texture and makeup.
Hopes this helps a bit anyway. If you have additional questions. I'll try to address them *LOL* but no promises I'll make sense...Mel
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Agate
Mar 13, 2009 12:08:47 GMT -5
Post by 150FromFundy on Mar 13, 2009 12:08:47 GMT -5
I try not to get too hung up on names and definitions. Technically, almost everything we cut, grind, and polish is some form of silicon dioxide (SiO2) with impurities. In fact, most of planet earth (that's not water) is SiO2 with impurities. This weak argument holds true weather your talking about quartz, aventurine, jasper, agate, coprolite, dinosaur bone, petrified wood, etc.
One leading rock expert may call a specimen agate, where another calls it jasper. A prime example would be "green tree agate". It looks nothing like agate. The base material is white and opaque. If anything, it is dendritic jasper, but we call it agate.
When in doubt, I simply tell anyone who ask that it's a lovely piece of polished silica, if I don't know the true identity. That way I don't have to admit that I don't have a clue what it is, or tell a lie. Maybe that makes me a generalist.
I don't think this debate will be resolved any time soon.
150FromFundy
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Agate
Mar 13, 2009 12:28:54 GMT -5
Post by Hard Rock Cafe on Mar 13, 2009 12:28:54 GMT -5
Thanks for finding the thread Steve! I knew we had talked about it recently, but I couldn't remember where and was too lazy to go digging.
Mel's previous response is still the best, IMHO, but I'm inclined to agree with 150, as well.
Chuck
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Agate
Mar 13, 2009 17:38:14 GMT -5
Post by Michael John on Mar 13, 2009 17:38:14 GMT -5
Maybe it's just a cop-out for my lack of knowledge, but I don't see the big deal about sticking a name on somewhat "common" rock ... unless maybe it's your pet rock. A lot of the monickers which are commonly used are generalities, at best. Ask 3 experts what an agate is and you'll get three different answers, and each answer will be at least partially based on opinion. There aren't clear cut definitions laid down by some governing body for this stuff, probably because these are "mutt" rocks ... quartz mixed with all sorts of different stuff. There are obviously popularly accepted generalities, but arguing or debating on a borderline quartz mutt's "geneology" is a sort of silly triviality. It's not like the rock is going into the Queen's Royal Jewels collection, it's just a rock. Make it into a pretty cab and get over it LOL.
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Agate
Mar 14, 2009 0:13:04 GMT -5
Post by mohs on Mar 14, 2009 0:13:04 GMT -5
Confucius said the beginning of wisdom starts with naming things properly. But then some wise guy brought him an agate and things have been confusious ever since
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Saskrock
fully equipped rock polisher
Member since October 2007
Posts: 1,852
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Agate
Mar 14, 2009 1:34:25 GMT -5
Post by Saskrock on Mar 14, 2009 1:34:25 GMT -5
I always just go with: Agate - light goes through it Jasper - light does not go through it and not white, grey or brown Chert - light does not go through it and it is white, grey or brown So far no one has said I'm wrong with this complex system so I'm sticking to it.
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Agate
Mar 16, 2009 10:36:15 GMT -5
Post by johnjsgems on Mar 16, 2009 10:36:15 GMT -5
There certainly is a lot of confusion. Agates are generally defined as chalcedony with random patterns and translucent if thin and back lighted. Jaspers are similar but opaque. A lot of what we find are "jasp-agates" or something in between. Chert is generally a solid color and the term gets used for any solid colored stone. Onyx is chalcedony with repeating patterns but a lot of onyx looking travertine is called onyx but should be "onyx" in quotes to denote it isn't. It's all perfectly clear.
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Agate
Mar 16, 2009 12:16:15 GMT -5
Post by Jack, lapidaryrough on Mar 16, 2009 12:16:15 GMT -5
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UtahRockHound
spending too much on rocks
Sometimes your the Windshield, sometimes your the Rock.
Member since May 2008
Posts: 301
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Agate
Mar 20, 2009 9:09:26 GMT -5
Post by UtahRockHound on Mar 20, 2009 9:09:26 GMT -5
Thanks everyone for the comments. I was hoping to add more content, but have been real busy.
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Agate
Mar 28, 2009 1:33:29 GMT -5
Post by mohs on Mar 28, 2009 1:33:29 GMT -5
According to the book I’m perusing Aptly entitled: One of the substance in a agate is a silicon- rich gel Which is quite jelly like ~mobile~ Anyway it gets real complicated But the gel contributes to the banding o the book as lots pics maybe not as good as this board but pretty nice!
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MikeS
fully equipped rock polisher
Member since January 2009
Posts: 1,081
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Agate
Mar 30, 2009 16:16:10 GMT -5
Post by MikeS on Mar 30, 2009 16:16:10 GMT -5
I suppose everybody has their own definition...like mentioned above- agate, chert, jasper, flint, onyx, ect are are closely related material, they just have different concentrations of certain minerals and are usually formed in different geologic processes. I have a similar rule of thumb as Saskrock...if I can see through it or a slice of it farily easily, I call it an agate, if not or if it's mixed with lots of other materials I call it jasper...
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Agate
Apr 1, 2009 13:45:16 GMT -5
Post by Michael John on Apr 1, 2009 13:45:16 GMT -5
Agate, according to Mindat:
A distinctly banded variety of Chalcedony.
The name is derived from its occurrence at the Achates River in southwestern Sicily.
Originally reported from Dirillo river (Achates river), Acate, Ragusa Province, Sicily, Italy.
Chalcedony, according to Mindat:
Traditionally defined as a fibrous cryptocrystalline variety of Quartz, more recently, it has been shown that much Chalcedony is a mixture of Quartz and Moganite, another silica mineral.
When it is concentrically banded (often in rather wild patterns) it is called by the subvariety name Agate. When it is in flat layers/bands it is called by the subvariety name Onyx.
[Note: Many non-banded forms of chalcedony - such as Moss agate - are often erroneously called 'agates'. True agate is concentrically banded. Mottled and included chalcedonies are more properly called simply 'chalcedony.' Petrified wood (agatized wood) is the name given to fossil wood where the replacement of the wood is by chalcedony, but the banding in this case is due to the wood structure - not concentric deposition of the chalcedony - and the material is chalcedony, not true agate.]
Jasper, according to Mindat:
A variety of Quartz
A dense, opaque, microcrystalline variety of quartz, usually red, brown or yellow and coloured by oxides of iron. It is used as a decorative stone.
The term is also used for other silicified rocks or decorative stones (in particular varieties of rhyolite are often termed Jasper).
*******
If their definitions are "correct", then it's no wonder that there's so much confusion. These definitions don't jive with popularly-accepted monickers which have been used for decades, or possibly even centuries in some cases.
If you were trying to sell some moss agate but you were calling it chalcedony (~correct term~), prosepective buyers would think you're an idiot.
Walk into any jewelry store and ask to see their onyx rings, and you'll be shown rings with black stones ... none of which are "true" onyx.
What about jasper? Just about any rock with quartz in it could be called jasper. All this fuss about agate, yet you can call almost anything jasper and nobody questions it.
Steve and I found "seam agate" when he was here, and I went back there a few days ago (affectionately called "Agate Road"). The material there is obviously from seams, but most of it has concentric, flat, and/or "crazy" banding, sometimes all 3 in one piece. So, "seam agate" seems to be the logical choice, since that's obviously what most would call it, but if you were to apply Mindat's definitions, the most correct term for this conglomerate would be chalcedony. BUT, if I were to call it chalcedony, I'd surely have people telling me it's seam agate. Catch 22.
I've read many people refer to chrysocolla-in-quartz as "agatized", yet I've never seen it in concentric bands. I've seen it called chrysocolla jasper, which is a correct terminology, but I think that would confuse most people a bit. In general, the single term "chrysocolla" is used for a wide variety of minerals found near copper, all mashed together, with the assumption being that there's quartz "in the mix". I won't even go into the deceptive abuse of the term "parrot wing".
IMO, there's just WAY too much focus on naming rocks, overall. We're not going to wake-up one day to find that the whole technical versus popularly-accepted naming mess has been fixed. Even discussing it is an exercise in futility, and pretty boring, too. If only we could all just call them "pretty rocks" and get-on with collecting them and shining them, we'd sure be making much better use of our time.
"What's in a name? That which we call a rose by any other name would smell as sweet"
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Agate
Apr 3, 2009 1:03:51 GMT -5
Post by mohs on Apr 3, 2009 1:03:51 GMT -5
At least the experts agree about the River Achates in Sicily & Pliny & Theophrastus (a.d.77) are credited with speaking of chalcedony
This is troubling !!!
What I want to know is: what came first? What comes first? Chalcedony or Agate?
Are they one in the same?
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MikeS
fully equipped rock polisher
Member since January 2009
Posts: 1,081
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Agate
Apr 3, 2009 12:21:43 GMT -5
Post by MikeS on Apr 3, 2009 12:21:43 GMT -5
to the best of my understanding, chalcedony is the primary componant of agate, but it can have other stuff in it as well...
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bushmanbilly
Cave Dweller
Member since October 2008
Posts: 4,719
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Agate
Apr 5, 2009 14:28:59 GMT -5
Post by bushmanbilly on Apr 5, 2009 14:28:59 GMT -5
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bushmanbilly
Cave Dweller
Member since October 2008
Posts: 4,719
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Agate
Apr 5, 2009 14:42:19 GMT -5
Post by bushmanbilly on Apr 5, 2009 14:42:19 GMT -5
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