christianswest
having dreams about rocks
Member since June 2023
Posts: 54
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Post by christianswest on Sept 7, 2023 7:32:13 GMT -5
I have an interest in agates. I see references to prairie agates and Montana agates. Some say these are not agates, but banded chert, jasper or chalcedony since they are not translucent. Then I go to The Quartz Page and see this:
A chalcedony is usually called an agate if it exhibits any of the following properties: -banding of whatever kind, caused by different colors or different structure of the layers, or both -translucency in conjunction with being multicolored - translucency in conjunction with a nodular shape and colored inclusions
An "ideal agate" by that understanding would be a nodule filled with a translucent, multicolored chalcedony with parallel bands. The minimum requirement would be that it is either translucent and exhibits some colored pattern or shows banding.
So my question is…is rock naming subjective depending on who’s doing it. A prairie agate might be an agate to one person, but not another? I see similarities in the plant world where “common” names can mean different plants in different areas of the country and that’s why the pros use the Latin names for accuracy.
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aGates
spending too much on rocks
Building a silver studio
Member since January 2021
Posts: 467
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Post by aGates on Sept 7, 2023 7:45:24 GMT -5
Hello! Good questions! Essentially translucent chalcedony with banding or inclusions (which don't usually hold the translucent character) is an agate. But chalcedony is the mineral name made up of micro crystalline quartZ and it can be expressed as jasper a non see through colorful due to the "impurities" aka minerals like iron and some of the others. Not sure the origin of sillica. Chert is basically jasper but the origin is a sea floor I believe.
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Post by vegasjames on Sept 7, 2023 7:59:37 GMT -5
There is the geological definition of agate and there is the loose definition of agate just like there is a geological definition of chrysocolla and the loose definition of chrysocolla where people call any unidentified copper mineral or ore "chrysocolla" leading to confusion. There are specific definitions for both.
Common chalcedony, agate, jasper and chert/flint are all chalcedonies, which are cryptocrystalline to microcrystalline quartzes all formed as the dehydration of opal.
True agates are defined as clear to translucent chalcedonies having curved multicolored bands. Therefore, a lot of stones referred to as "agate" such as "moss agate", "plume agate", "dendritic agate" and even "waterline agates" are not true agates. They are chalcedonies. Most people have a hard enough time trying to pronounce chalcedony (kal said uh nee) properly as is. so adding moss, plume, dendritic or waterline before chalcedony will not help. Therefore, a lot of people simply refer to any chalcedony with some clarity or translucency as "agate" even though it incorrect.
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Post by vegasjames on Sept 7, 2023 8:12:43 GMT -5
Hello! Good questions! Essentially translucent chalcedony with banding or inclusions (which don't usually hold the translucent character) is an agate. But chalcedony is the mineral name made up of micro crystalline quartZ and it can be expressed as jasper a non see through colorful due to the "impurities" aka minerals like iron and some of the others. Not sure the origin of sillica. Chert is basically jasper but the origin is a sea floor I believe. Yes, chert/flint have their origin in the sea floor.
As with all chalcedonies, they form as opal loses water and crystallizes. Opal starts as a concentrated silica solution. The silica molecules start to move closer together squeezing out water and forming a liquid gel. As the molecules continue moving closer together, more water is squeezed out. When the water content finally reaches 23% a solid gel forms known as opal. Therefore, opal is not a true solid and it is amorphous (lacking crystal structure) like glass. As the water content drops further, 2 things happen. The opal gains density and when the water content drops below 3% the opal starts to crystallize in to some form of chalcedony.
I mention the increase in density because this is one of the easy ways to identify opal. Opal feels rather light compared to chalcedony, which in some cases can look and feel like opal. If the opal is in transition then this become harder as the stone is now partially opal and partially chalcedony and thus too dense for opal and not dense enough for chalcedony.
The silica source plays a role in whether the opal formed will convert in chert/flint or a different form of chalcedony.
Chert/flint forms from opal that was formed by biogenic silica sources such as diatoms, radiolarians, etc.
Other forms of chalcedony form from inorganic sources of silica such as sandstone or clays.
Opals can also pick of varying amounts of aluminum oxide from sources such as clays and diatoms that affect the harness, stability and density of the opals.
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hypodactylus
spending too much on rocks
Member since July 2021
Posts: 434
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Post by hypodactylus on Sept 7, 2023 10:09:34 GMT -5
I like vegasjames's explanation/definition, as that is how I have come to understand the differences. Rock naming is definitely somewhat 'subjective' and inconsistent. Not to mention, you will often see people completely misidentifying types of rocks. You will see 'jaspers' that aren't jasper, 'agates' that aren't agate, etc. You will also see people mis-label rocks online, probably thinking they can fool someone into paying more.
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christianswest
having dreams about rocks
Member since June 2023
Posts: 54
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Post by christianswest on Sept 7, 2023 14:17:52 GMT -5
Thanks aGates, vegasjames, and hypodactylus. All this is very helpful and explains things very well. I learned something new today about how they are formed.
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Post by Son Of Beach on Sept 10, 2023 20:49:01 GMT -5
Yes, chert/flint have their origin in the sea floor.
Chert/flint forms from opal that was formed by biogenic silica sources such as diatoms, radiolarians, etc.
Good to know James
This is a small piece of Ohio Flint I found when doing cleanouts. Pic doesn't show it well, but they are there.
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fuss
spending too much on rocks
Member since October 2018
Posts: 250
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Post by fuss on Sept 24, 2023 7:28:18 GMT -5
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Post by vegasjames on Jan 18, 2024 1:29:42 GMT -5
Yes, chert/flint have their origin in the sea floor.
As with all chalcedonies, they form as opal loses water and crystallizes. Opal starts as a concentrated silica solution. The silica molecules start to move closer together squeezing out water and forming a liquid gel. As the molecules continue moving closer together, more water is squeezed out. When the water content finally reaches 23% a solid gel forms known as opal. Therefore, opal is not a true solid and it is amorphous (lacking crystal structure) like glass. As the water content drops further, 2 things happen. The opal gains density and when the water content drops below 3% the opal starts to crystallize in to some form of chalcedony.
I mention the increase in density because this is one of the easy ways to identify opal. Opal feels rather light compared to chalcedony, which in some cases can look and feel like opal. If the opal is in transition then this become harder as the stone is now partially opal and partially chalcedony and thus too dense for opal and not dense enough for chalcedony.
The silica source plays a role in whether the opal formed will convert in chert/flint or a different form of chalcedony.
Chert/flint forms from opal that was formed by biogenic silica sources such as diatoms, radiolarians, etc.
Other forms of chalcedony form from inorganic sources of silica such as sandstone or clays.
Opals can also pick of varying amounts of aluminum oxide from sources such as clays and diatoms that affect the harness, stability and density of the opals.
I was just reading that diatoms are Jurassic age, or at least thats our earliest evidence. Is it just assumed that they were also around earlier or is there a more specific species that would have been around before them to make the pre-jurassic cherts? Oh, the radiolarians? Are there more? Not sure, but we are still talking many millions of years, which should be more than sufficient time for chalcedonies such as chert to form.
It is a very common myth that is takes millions of years for just the opal to form, but it has been proven that opal actually forms very rapid. Opal can be grown and stabilized in as little as a year. So, add a million years or two and I would think that would be more than sufficient time for the opal to convert in to a chalcedony.
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RWA3006
Cave Dweller
Member since March 2009
Posts: 4,208
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Post by RWA3006 on Jan 18, 2024 8:25:04 GMT -5
I was just reading that diatoms are Jurassic age, or at least thats our earliest evidence. Is it just assumed that they were also around earlier or is there a more specific species that would have been around before them to make the pre-jurassic cherts? Oh, the radiolarians? Are there more? Not sure, but we are still talking many millions of years, which should be more than sufficient time for chalcedonies such as chert to form.
It is a very common myth that is takes millions of years for just the opal to form, but it has been proven that opal actually forms very rapid. Opal can be grown and stabilized in as little as a year. So, add a million years or two and I would think that would be more than sufficient time for the opal to convert in to a chalcedony.
Good insights on the subject, James. Another thing to remember is the Jurassic is relatively recent on the overall timeline of the earth while seas were present. I've found marine origin chert layers almost a thousand feet lower than the Jurassic layer above them.
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Post by velodromed on Jan 28, 2024 9:02:34 GMT -5
Hello! Good questions! Essentially translucent chalcedony with banding or inclusions (which don't usually hold the translucent character) is an agate. But chalcedony is the mineral name made up of micro crystalline quartZ and it can be expressed as jasper a non see through colorful due to the "impurities" aka minerals like iron and some of the others. Not sure the origin of sillica. Chert is basically jasper but the origin is a sea floor I believe. Yes, chert/flint have their origin in the sea floor.
As with all chalcedonies, they form as opal loses water and crystallizes. Opal starts as a concentrated silica solution. The silica molecules start to move closer together squeezing out water and forming a liquid gel. As the molecules continue moving closer together, more water is squeezed out. When the water content finally reaches 23% a solid gel forms known as opal. Therefore, opal is not a true solid and it is amorphous (lacking crystal structure) like glass. As the water content drops further, 2 things happen. The opal gains density and when the water content drops below 3% the opal starts to crystallize in to some form of chalcedony.
I mention the increase in density because this is one of the easy ways to identify opal. Opal feels rather light compared to chalcedony, which in some cases can look and feel like opal. If the opal is in transition then this become harder as the stone is now partially opal and partially chalcedony and thus too dense for opal and not dense enough for chalcedony.
The silica source plays a role in whether the opal formed will convert in chert/flint or a different form of chalcedony.
Chert/flint forms from opal that was formed by biogenic silica sources such as diatoms, radiolarians, etc.
Other forms of chalcedony form from inorganic sources of silica such as sandstone or clays.
Opals can also pick of varying amounts of aluminum oxide from sources such as clays and diatoms that affect the harness, stability and density of the opals.
Thought I’d pull up this post from last year and ask where jasper fits in the picture. It is really just chert with iron oxide or other mineral staining or is there more to it?
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Post by vegasjames on Jan 28, 2024 22:51:37 GMT -5
Yes, chert/flint have their origin in the sea floor.
As with all chalcedonies, they form as opal loses water and crystallizes. Opal starts as a concentrated silica solution. The silica molecules start to move closer together squeezing out water and forming a liquid gel. As the molecules continue moving closer together, more water is squeezed out. When the water content finally reaches 23% a solid gel forms known as opal. Therefore, opal is not a true solid and it is amorphous (lacking crystal structure) like glass. As the water content drops further, 2 things happen. The opal gains density and when the water content drops below 3% the opal starts to crystallize in to some form of chalcedony.
I mention the increase in density because this is one of the easy ways to identify opal. Opal feels rather light compared to chalcedony, which in some cases can look and feel like opal. If the opal is in transition then this become harder as the stone is now partially opal and partially chalcedony and thus too dense for opal and not dense enough for chalcedony.
The silica source plays a role in whether the opal formed will convert in chert/flint or a different form of chalcedony.
Chert/flint forms from opal that was formed by biogenic silica sources such as diatoms, radiolarians, etc.
Other forms of chalcedony form from inorganic sources of silica such as sandstone or clays.
Opals can also pick of varying amounts of aluminum oxide from sources such as clays and diatoms that affect the harness, stability and density of the opals.
Thought I’d pull up this post from last year and ask where jasper fits in the picture. It is really just chert with iron oxide or other mineral staining or is there more to it? Jasper and chert/flint are in essence the same thing. They are both opaque, impure chalceodonies. The main difference is in the formation. Jasper is produced from the dehydration of opals formed from inorganic silica sources such as clays and sandstone as where chert/flint is formed from opal that was produced from biogenic sources of silica such as diatoms and radiolarians.
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Post by velodromed on Jan 29, 2024 7:30:21 GMT -5
Thought I’d pull up this post from last year… Jasper and chert/flint are in essence the same thing. They are both opaque, impure chalceodonies. The main difference is in the formation. Jasper is produced from the dehydration of opals formed from inorganic silica sources such as clays and sandstone as where chert/flint is formed from opal that was produced from biogenic sources of silica such as diatoms and radiolarians. That’s what I was wondering! So cool… Many thanks James!
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