What is in the center and on the ends of this chalcedony nodule and how was it formed
Looking at the images, and comparing it to other known samples of chalcedony, I am inclined to believe that it falls into 1 of 3 possibilities:
(and possibily the least likely) There may have been a localized mass of chromium present when this sample formed. The greenish-grey portion is reminiscent of Aventurine, which is chalcedony which often times contains Chrome-bearing fuchsite. I would like to know if any optical phenomena might be present if that section was cabbed and polished. Aventurine also forms in brownish colors as well, and the images seem to show a highly reflective shimmer within the brown sections.
A variant of 1. Chalcedony is capable of forming psuedomorphs, ie: replacing one mineral with another, in this case, the chalcedony replaced what was originally around the core material green/copper material. The core material might not even have to be “native” to the geologic processes that formed the chalcedony. Chalcedony has been known to replace various shells and other forms in the pseudomorph process, much the same way opal, hematite, and pyrite has been known to do.
the core material simply served as a seed point that the chalcedony was able to latch onto to begin its growing process. Chemical reactions between the seed and the solution that the chalcedony was suspended in may have resulted in the color-shift in the seed.
Chalcedony has a good degree of solubility in water, so its strongly possible that the sample you have was formed via hydrous deposition. at 100C, you can have up-to 40mg per liter of the microcrystaline silica dissolved in water, and as the water evaporates out, the chalcedony would gradually build up, surrounding whatever is present at the time of formation. This mechanism is how chalcedony is able to form pseudomorphs & may also potentially use a non-chalcedony nucleus for a seed.
What type of chalcedony are the seed materials made of ? During the hydro process wouldn’t the inclusions have mixed somewhat? What would be a test I could do to specify the exact classification of chalcedony of this stone? I am cabbing the stone and doing a light test, and will give the results in a few days. thanks Paul
As far as testing goes, I would suggest using the second slice down in picture 2. Separate and flat polish a sample of just the core material, and test its refractive index and specific gravity. These two tests should greatly narrow down the field as to what the core material may be comprised of.
The seed may not be chalcedony at all. All that the seed needs to do, is provide a surface for the chalcedony to anchor onto. This can be almost anything present within the growing environment. Even dinosaur bone has been used as nature’s seed material.
It would be helpful to know the origin of the specimen. That may help inform to the seed composition.
What I find interesting, is how “egg-like” that nodule appears to be. While a “petrified egg” is a flight of fancy, Agate forms of Chalcedony are known to petrify wood, and Silicified dinosaur bone has been discovered. An analysis of the structure of the core may provide some interesting insights.
There are so many different forms of chalcedony, that narrowing it down to a single variant would be difficult. Agates have translucency while jasper is opaque. Both are forms of chalcedony. And then there are hybrids referred to by some has jasper-agates.
The grayish outside is suggestive of chert, and the core could be Aventurine. (Opaque, compact Quartz / Chalcedony containing small Mica, Hematite, or Goethite scales which cause a glistening effect. Aventurine is most often green but may also be other colors such as gray, orange, and brown. -source- http://www.minerals.net/gemstone/chalcedony_gemstone.aspx). I am not able to see if there is any shimmer from the images.
All chalcedony will have similar testing results. The only way to truly identify the core, is if it comes back as something other than chalcedony.
Thank you for your amazing insight, the egg shape of the nodule is part of a large collection of ancient tools that when tested at the nys museum came back chalcedony with fossils in the matrix and origin unknown, the archaeological research points to a group of humans that carried these stones across the land bridge, as science crosses from field to field so often we find new discoveries. The latest discovery includes insight into the lives of this ancient tribe as they used these chalcedony nodules in a cameo type process rubbing pictures onto the bifaces of the stone.
Shame that the origin is unknown.
I am still curious about the core of the one sample. IE< if it is a fossil, what creature did it come from? was it a plant fossil, animal fossil, or something else entirely. The core is so visually distinct from everything else, that I think it might be worth a little further digging, if for no other reason than plain curiosity.
While I am sure the NYS museum has some fantastic tools, I would like to have those results compared to that of a GIA or similar caliber gem lab. a high-end gem lab can get down to the minute crystallography, and may even be able to determine origin. Some of the ways this can be accomplished is through a study of the minute impurities within the sample. These impurities often act as an origin fingerprint. However, such testing maybe prohibitively expensive.
After cabbing an end and center piece it is clear that the red and blue do not carry light across its face but seem to change and become darker with light, these materials are opaque, yet the outer shell shines up nicely but without shimmer or translucency. I still am far away from an identification. I thought showing some of the other nodules would help with clues they are all chalcedony and must of come from the same place at some point. Can corundum change from red to blue if that was what it is?
While there is color change corundum, it is very rare, and corundum is significantly harder than chalcedony. While there are a fair number of color-change stones, the common color change from blue is typically to purple. The other common shift is green to red. I have not seen or heard of a blue/red shift.
Chalcedony is a common stone, and can be found almost anywhere there is quartz. The only way to determine origin is to finger-print the inclusions and hope the inclusions are particular to a region.
Obtaining the refractive index and specific gravity of the core material will go a long way in identifying what it is. Without those two values, we are just throwing darts at a wall we cannot see.
There was a typo when I said it did not take a shine when clearly it does, sorry.
I was also hoping someone can comment on the ancient hammerstones where the finger divots weren’t the only areas rubbed revealing the images you see in a cameo effect style, would this be considered jewelry or art? What are the first examples of ancient chalcedony being used for jewelry or art? These examples traveled across the great land bridge and ancient people suffered a catastrophe and is probably why they turned some of their basic hammerstones into these picture message stones as I call them. There is also a half circle cut in many of the stones as deep as an eight of an inch and up to one inch long and I was wondering how big the faceted diamond or other primitive chisel would have to be to make that cut in these chalcedony nodules, and how long would it take to rub out a finger divot in one as well. Even though they usually only made finger divots in the softer fossil in the matrix? All of the samples have been authenticated by several museum curators, and any answers or contributions could be made part of the ongoing research.
I am honestly not sure. Paleontology and archaeology are outside of my ken so to speak.
Thank you for your reply!