Advertisement

Green Chert?

topic posted Wed, October 12, 2005 - 9:24 AM by  Captain
Share/Save/Bookmark
I recently purchased a small polished stone from a merchant, who described it as Green Chert.

Never having heard of Chert, I tried doing some web research, and found conflicting information.

On one site, the implication was that Chert was a category that included a number of different stones including Flint.
On another site, the author stated that Chert was Flint.

Can anyone clarify for me what Chert is, what is its connection with Flint, and also what I may have in my possession...what is Green Chert?
posted by:
Captain
Australia
Advertisement
  • Re: Green Chert?

    Mon, October 17, 2005 - 10:12 AM
    Hi Zane,
    I grew up in Missouri and the name "chert" was commonly used (as was flint) to describe the same, somewhat dull, cryptocrystalline quartz rock.

    Cryptocrystalline quartz (CQ), as you may know, is quartz (or silicon dioxide) whose individual crystals have assembled in a non-uniform fashion. Whenever small crystals of a mineral assemble in an orderly fashion, the result is a crystal, which often reflects the symmetry of the individual crystals. CQ is merely massive quartz - lots of quartz crystals in a random arrangement. The only couple, obvious identifying characteristics of CQ are its hardness and that it typically shows conchoidal fractures. Enough of the geology lesson and apologies if you knew this all.

    Anyway, CQ is extremely common as you know, and goes by many names (including agate, chelcedony, opal, jasper, flint and chert).

    Many of these names are somewhat subjective but sometimes reflect the color of the stone (onyx is almost always black - though it may have been dyed, bloodstone is dark green with red flecks, carnelian is reddish, etc.).

    Another major distinction in CQs that helps in naming is the purity, or amount of silicon dioxide, and grain size. A general rule of thumb i've encountered is, the more pure the silicon dioxide content and the smaller the grain size, the more waxy the natural luster. Conversely, the less pure and larger grained CQs tend to have a duller natural luster.

    Chert is the common name for a less pure, larger grained CQ. Flint is merely a type of chert. Both tend towards dull colors such as black, cream, brown or white, and are less common in pure colors of green, red, orange, etc. In some schools, however, the term flint is applied towards shinier, black cherts. Again, in the world of CQs, categorization is less a science and more a localized tradition.

    Finally, most CQs will take a nice polish, whether hand or wheel polished, or tumbled. It is 99% probable that you have a piece of CQ. If your stone seems highly translucent, it might even be dyed agate (it's very common to dye agates). If it is more opaque, then it could be considered colored chalcedony. Besides bloodstone, the most commonly named green CQ is chrysoprase, which tends to have an even, bright apple-green color.

    Bottom line, though - chert and flint are basically the same thing and tend to be the names used for more common CQ that is less likely to be polished or used in jewelry. If your piece has a really nice polish and has any amount of translucency, it's probably agate or chalcedony. But don't give the merchant a hard time. Few people can look at a polished stone and give it a proper name!

    Hope this helps!
    • Re: Green Chert?

      Mon, October 17, 2005 - 10:26 AM
      Thank you, dexter, for that detailed and clear reply.

      The stone is totally opaque, but has a nice polish to it. It is basically very dark green, but gradually shifts to an almost jade green color as you move along the surface of the stone.

      I don't know where the merchant got the stone, but I certainly wouldn't give him a hard time. My question was more for my own edification.

      Currently living in Ohio, I have found chunks of flint quite often. And since this stone did not resemble the native flint in any visible way, I was surprised to see that the two are basically the same.

Recent topics in "rocks and minerals"

Topic Author Replies Last Post
gem and mineral shops in NYC Josh 7 May 8, 2014
New member check out the pics john 1 October 30, 2009
Ooops! BJ 5 October 12, 2009
What kind of stone is this? BJ 0 October 12, 2009