EARTH ROCK

[roustabout149]

This is one of the rocks I found in the field this year. I never leave a fascinating rock behind and finding this particular piece is one that keeps me searching!

[itwasluck]

That is a pretty piece there Pam. I dont know much about materials but do you think it might be part of a geode?

[roustabout149]

itwasluck wrote:

That is a pretty piece there Pam. I dont know much about materials but do you think it might be part of a geode?

  Thank you.  I also find the piece to be pretty.  In answer to your question, "Yes, absolutely";  part of a geode that is.  Chalcedony and quartz crystals would be my guess as far as the material is concerned.  Now, for as much as I do not understand where a lot of your comments are coming from, this one I understand!!!  Progress!!!  Thanks again.
Pam

[painshill]

You’re pretty much spot-on. Often rocks are a mixture within the family that they come from and this is particularly true for the quartz family… which is what we have here. When this is the case, we often use the term “trend” or “trending”. So that looks to be chalcedony, but trending to agate or more likely moganite (not to be confused with morganite) in the areas that are more translucent. Moganite is essentially quartz but with a different crystal habit. The truly crystalline areas are quartz and when it appears in coatings and the linings of vugs like this, we call the resultant mass a “druse” or refer to the rock as “drusy” (the American spelling would normally be with a “z”, not an “s”).
It’s not a geode. Geodes differ from vugs in that they form as a separate entity from the surrounding rock. They have a “shell” of chalcedony and a mineral interior which is usually quartz or a derivative thereof. Vugs are voids or cavities within a rock formation, as you have there.
So, if you wanted to be geologically precise, I would say it’s a chalcedony trending to moganite with quartz druse (and also very pretty). The black speckles are interesting. Don’t know what those are and I can’t tell from just a picture.

[roustabout149]

Thank you very much, Roger.  I am going to copy this response and add it to the description of the rock in my photo album.  If you have any reservations with my doing so, just let me know and I will remove it.  A vug, huh? :huh: A vug is precisely what creates the, "eye", of the Bison!  Don't ya' find it uncanny how that eye is centered in the material! :laugh: There's more to the Bison than meets the eye of experts, IMHO.  There's nary an expert, or anyone else for that matter, who can rain on my parade! So be it.

[painshill]

Pam
You're most welcome to the descriptiption I offered. Chalcedony is characterised as an intergrowth of quartz and one of its polymorphs known as moganite. Your rock seems to be mostly chalcedony (the mixed intergrowth) but also with the two subcomponents present in isolated recognisable form. Think of it like having a piece of fudge cake where you can also see blobs of butter and crystals of sugar that aren't properly mixed in. The blue/gray glassy translucent areas are what I believe to be moganite. Here's a chalcedony specimen with a much larger expanse of isolated moganite:

[re the bison, I wouldn't dream of raining on your parade! You see what you see]
Roger

[SDhunter]

Didn't I see a Hertzian cone on that first rock.  Thanks alot Chris for getting
me started on Hertzian cones!!  LOL
  Gary