What stone is that? – for Mrs. Yollis and Class

I received a question via Twitter...

Do you know what kind of stone this is ?

There was the below photo attached to the tweet...

This photo was supplied by Mrs. Yollis and class.

This photo was supplied by Mrs. Yollis and class.

I like a challenge and, although not always successful, finding an answer. I have an interest in geology but l find I know a little about many things but not a lot about anything. Without being able to hold the rock and look more closely, and without expertise, my first stop was to look more closely at the photo. Here's what I noticed...

The stone was white to bluish-grey.

Breaks around the edges looked a little like they might break off in flattened, sharp edged pieces.

There appears to be an inner border (lining) on the stone.

I wanted to see the border closer so I enlarged a section of the photo (below). I also enhanced contrast and colour a little.

This photo was supplied by Mrs. Yollis and class.

This photo was supplied by Mrs. Yollis and class.

 I now noticed there seems to have been some fine layering around the edge of the stone up to the border. Layering can mean sedimentary rock but, it can also be a sign of a space in rock filling with crystals. I had seen something like this before. I probably have a few hundred small mineral samples in my collection so I started to search. Firstly, a display of stones I had used with classes over the years...

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

A few of these showed some of the features I was looking for, especially calcite, agate and quartz.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

This reminded me of larger samples I had in my minerals database, I found items 41 and 46. They are listed as "Quartz - Chalcedony - Agate". They are examples of silicon oxide (SiO2).

The first sample has been cut and polished. The layering towards the outer edge is easy to see. There are small quartz crystals in the centre of the sample. I have seen this in other of my samples where inner spaces aren't completely filled. See geodes and the additional photos at the end of this post.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

The second sample was a piece broken off a larger sample and only had a low sheen.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Without being able to hold and examine the sample in Mrs. Yollis's class, I suspect they have a sample of agate or chalcedony. Remember, I am only interested in geology and not an expert so I'm really only guessing.

How is it formed? My database explains it this way...

A concentric, banded, fibrous variety formed by precipitation from watery solutions in rounded cavities in lava rocks (geodes), sometimes with beautiful clusters of rock crystals or amethyst at the centre.

From my collection, below are photos of geodes. Most have been cut and one polished to show the interior.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

Schools and students have permission to use this graphic for non-commercial, educational purposes.

The last photo shows one cut sample and an uncut geode beside it. What's inside the uncut geode? I can tell you it is about the same size as the cut geode if it was whole. This means the uncut geode should be about twice the weight of the half but it is around three times the weight of the half. It may be solid or have a small central cavity. It could be very beautiful or possibly plain inside. We'll never know because I won't have it cut. I like a little mystery in the world. 🙂

4 thoughts on “What stone is that? – for Mrs. Yollis and Class

  1. Heather

    Dear Mr. Mannell,

    Thank you for sharing this post to us!

    I think Keira’s rock looks like the Calcite, Quartz Crystal, Geodes and the Agate all combined together. The outside of the rock looks like a Microcline. She found it at a campsite when she was camping, so I wonder how it got there. Do you know how it got there?

    On your chart, there is a rock called the Azurite. I took a glance at it and to me, it seemed like the rock was a pattern of Earth! Where can you locate the rock in Australia?

    I have a crystal that looks almost like the purple crystal that was cut in half. My crystal is blue. What kind of crystal is the purple one?

    You explained about how the crystals form. I am a little confused with your explanation of all of the high scientific words. Can you clarify what do they exactly mean?

    How do you know so much about minerals?

    Your friend from last year and this year,
    Heather

    Reply
  2. rossmannell

    Post author

    Hello Heather,

    Your comment is stunning in its clarity and suggestions. One key importance in science is the interaction of peers when trying to find solutions. As we are both learners in geology, we’re peers so such a quality comment deserves a quality comment.

    I am working on a post (about Australia) for someone else at this time but I will prepare one for you next. Hopefully it will be up before the weekend because I have a very busy few days starting on Saturday afternoon.

    I will send the link to your class blog when done. 🙂

    Ross Mannell

    Reply
  3. Keira

    Dear Mr. Mannell,

    Thank you for posting information about my rock!

    I agree with you that the rock looks like a combination of Calcite, Quartz, Geodes, and Agate. The outside of my rock looks like an amber color. Do you know why it looks amber on the outside?

    On your rock chart, there is a rock called Galena. To me, Galena looks like a rock wrapped in shiny tinfoil. Where is Galena most common?

    In the photos of Geodes, the first one looks interesting to me. The middle almost looks like it is hand painted. Was the middle polished or was it like that when you discovered it?

    The last picture in the post also looks interesting. In the middle of the rock that is cut open it looks like there is a little nook. Am I right? It almost looks like there is sand in the little nook! I know there is not sand in there though.

    How do you find out all these facts? Where did you get all these rocks?

    Your rock-loving friend,
    Keira

    Reply

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