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Identity HelpFlint or agate?

25th Aug 2016 18:05 UTCJennifer Cindrich

I live in central Texas in the llano uplift. There is a lot of flint around but sometimes I find these different ones with an obvious outer looking shell and banding in the center. Is it an Agate? also, If I found one with no banding how could I tell the difference between onyx and flint.

Thank you.

25th Aug 2016 18:19 UTCAlfredo Petrov Manager

It's probably chert.

http://www.mindat.org/min-994.html

25th Aug 2016 20:05 UTCAmir C. Akhavan Expert

Banded flint.

Or banded chert, if you wish.

25th Aug 2016 22:49 UTCBob Harman

So we use the terms AGATE, CHERT, FLINT, and CHALCEDONY. Are they essentially interchangeable or, within limits, one and the same?

What I mean is that are all basically variations on microcrystalline quartz? Is the basic term chalcedony and if it is a small nondescript piece then can it be called either flint or chert. And, if it is a larger piece with variously colored bands then can it be called (a piece of) an agate? For smaller pieces, other than a well formed easily recognized true "agate", is there any REAL importance in being specific and separating one from another?

Thanks for your help. CHEERS.......BOB

26th Aug 2016 00:00 UTCJennifer Cindrich

the reason I would want to tell one from the other is so that I can hunt for an agate when agate hunting and flint/chert when I hunt for that. what are the differences if any?

This piece is as big as a size 15 shoe or football.

26th Aug 2016 02:26 UTCDoug Daniels

Agate, chalcedony, chert, and flint are all cryptocrystalline (micrcoscopically crystalline) quartz. From my experience/training, agate and chalcedony tend to be translucent (they can pass light); agate is banded, usually in various colors but not always (you may have just whitish and grayish bands). Guess you could say agate is a banded chalcedony. Most things I've seen labelled as chalcedony tend to be white, whitish, to gray or grayish in color, and does not show banding. Chert and flint tend to be opaque, with chert usually being gray in color, flint black or dark brown (basically very dark in color). The line between chert and flint is ... iffy.

26th Aug 2016 02:41 UTCBob Harman

Thanks DOUG. About as I expected. Somewhat blurred lines between them, but there are subtly recognizable differences in most examples. CHEERS.......BOB

26th Aug 2016 02:44 UTCPaul Brandes 🌟 Manager

Sorry to say, I don't believe you're going to find any agates on the Llano Uplift. Agates usually form in volcanic basalts which the Uplift itself is lacking. For good agates, you'd have to travel to the Big Bend area. As far the differences between agate and chert/flint, Doug is pretty much spot on.

26th Aug 2016 04:25 UTCJennifer Cindrich

The llano uplift does indeed support agstes. correct me if I'm wrong-they can occur in tuffs (an extrusive volcanic rock) Paul, that of which I have an abundance of in my yard.

26th Aug 2016 04:48 UTCDoug Daniels

As I remember the geology of the Llano Uplift.....it is mainly Precambrian aged, granitic rocks. Nothing volcanic - then again, been a long time since I've considered that area. There may be some areas that harbor agates....like they say, "don't mess with Texas." But as Paul mentioned, you're more likely to find agates down in the Big Bend area, where there's lots of volcanics.

26th Aug 2016 04:59 UTCJennifer Cindrich

nope. you should come give us a visit. There has been plenty of volcanic activity here in my little spec in central Texas.

26th Aug 2016 05:03 UTCJennifer Cindrich

26th Aug 2016 05:43 UTCDoug Daniels

Oh...THAT part of the Llano.....like I said, a long time since I visited the area......

26th Aug 2016 11:22 UTCJennifer Cindrich

you all have been so helpful! I love this board and exercising my brain in this manner, so fun!!

26th Aug 2016 15:35 UTCEd Clopton 🌟 Expert

This area of terminology is a mess, not unlike some other areas of geological terminology. The confusion results from 19th- to 20th-century attempts to impose scientific order on traditional-to-ancient names that were applied before there was (or more recently by people who didn't have) a shared chemical understanding of the relationships among rocks and minerals.

My understanding is that chalcedony is a mineral (a relatively pure cryptocrystalline variety of quartz, usually translucent to transparent); agate is chalcedony having parallel or concentric bands of contrasting color and/or transparency*; chert is a chemical-sedimentary rock consisting of chalcedony plus one or more impurities that render it opaque or nearly so; flint is a grey to black variety of chert; and, while we're on the subject, jasper is an attractively patterned or colored (especially red) chert**.

*Agate traditionally is applied also to certain non-banded varieties of gem chalcedony such as moss agate and plume agate.

**Jasper is also applied in the lapidary world to various attractively colored or patterned non-chert materials, such as "chicken-track jasper", which is a form of rhyolite.

Jennifer, it's less a matter of what you're hunting for than of what you find while out hunting. If I stumble onto a nice agate while out hunting for chert I won't leave it behind just because it's not what I'm after!

26th Aug 2016 21:47 UTCAmir C. Akhavan Expert

We do have an agate page on Mindat that explains what agate is.

If you study a petrographic thin section of banded flint and compare that with what you see in a thin section of agate, you will see a striking difference. Likewise jasper or various types of chert.

The differences between the various types of chalcedony are not that difficult to undertstand (even if their genesis is not fully understood).

The naming is only a mess because people- mostly dealers- name a lot of things either jasper or agate, for obvious reasons.

26th Aug 2016 23:00 UTCPaul Brandes 🌟 Manager

jennifer cindrich Wrote:

-------------------------------------------------------

> The llano uplift does indeed support agstes.

> correct me if I'm wrong-they can occur in tuffs

> (an extrusive volcanic rock) Paul, that of which I

> have an abundance of in my yard.

Jennifer,

Could you please post a photo of these "tuffs" you have in abundance? When I consult the USGS geologic maps of the area between Johnson City and Marble Falls along US Hwy 281, all I see is a lot of Glen Rose Formation (limestone/dolostone) with some associated limestone units. In NW Blanco Co., there is Town Mtn. Granite, the Hickory Sandstone, and a tiny piece of the Coal Creek Serpentinite. However, there are no basalts/tuffs in Blanco Co., and the closest true volcano (and associated pyroclastics) is southeast of Austin near McKinney Falls. There are also scattered mafic rocks in Gillespie and Llano Counties, but these are diorites and gabbros; neither of these produce agates.

7th Mar 2018 19:50 UTCBob Ehlert

I know this is a couple years old now but here are 2 agates I found in the dry riverbed of the Llano River downtown Llano

30th May 2018 16:19 UTCJennifer Cindrich

Thank you Bob

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