Wednesday, December 6, 2017

Sagenite

A rock that insists on telling you a story is a great rock to get to know!

Sagenite loves to tell stories. At least the kind I work with in my lapidary shop does. There are apparently two schools of thought when this name is used to describe a rock. There's the one that describes stones with vibrant, puffy strands that appear to be moving with an unseen tide. I absolutely love these. The other is a variety of rutile  where the crystals criss-cross  in a geometric sort of pattern. The look is so distinctive that back in 1796, it was named sagenite  from the Greek and Latin word "sagena" or net.

This is a link to a picture of the criss-cross pattern. https://goo.gl/images/m2aBpA

As a professional writer, I'm leery of copying someone's image for my use, like this blog, without permissions. This image appears on a site that says the images are royalty free, but then has a notice to check for copyright. I decided a link was the best way to go.


 I fell in love with the slab I used to create this cabochon!  It has the yellowish sagenite, along with white tube structures you can just see inside the clear agate, and a vug that comes across as a whirlpool that is causing the sagenite to swirl around.

Pat McMahan, renowned agate expert, wrote that these sagenetic filaments are often arranged in fans or sunbursts in the agates and can come in different colors. That is because the sagenite in agates is a pseudomorph (where one mineral replaces a different one that has already created the shape). As an interesting aside, he feels that the sagenetic structures form in the still-hardening agate and they don't extend into nearby banded agate which is formed at a different time.


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This cool piece of sagentic agate really shows off the way the filaments can arrange themselves in groups of fans.

The way it has a central open space in the agate with the filaments swishing around it, makes me think of that sight you see when you're in your car as it goes through the automatic car wash. All those soft things rotating and slapping your car as you go through.


The vendor who sold me this had a lot of slabs of this material. I asked him what it was and he wrote "sagenite" on the slab I purchased.

I've had other people who have seen it agree that it is sagenite, but it's different from my other pieces because there's no cluster that the formations spring from.

However, I love the look like it's from a pot of spaghetti and it polishes like a dream!





 Do you have a favorite agate cabochon that features sagenite?  I'd love to see a picture of it!

ROCK SHOW SEASON IS ON!

If you have a favorite club-based rock show, drop me a note and tell me why it's wonderful and maybe I can use it in a future blog!

Until next time, have fun with your rocks!

Your Lapidary Whisperer

Donna

Wednesday, November 22, 2017

Thanksgiving with Dendrites


 

 

 I've always loved dendrites in my rocks. Dendrites are markings in stones that look like ferns or trees or other plants--if you catch them along their long direction. However, when you cross-cut them, you get a bunch of dots like the two different angles of the same piece of dendritic opal below.

    

 

 

 

 

   

 

DENDRITE OR PLUME?

In today's blog, I'm taking a strict interpretation of dendrites. Some people like to include plume agates with dendritic agates,but aside from the fact that they look very different to me, Roger Pabian, co-author of "Agates: Treasures of the Earth" has clarified the difference in that he states that Dendrites form inside existing agates and Plumes form first and then are surrounded by agate. 

 

  In this picture, the oval on the left is dendritic opal, the oval on the right is plume agate. Notice how the plumes look a bit fluffy?

 

 

 

 

 

It is further complicated by the fact that not all rocks with dendrites are agates. However, they usually are made of chalcedony, a term used to describe a group of microcrystalline silica-based minerals that include quartz, jasper, and onyx. They are great for lapidary work all coming in at 6.5 to 7 on the Mohs scale of hardness. Another mineral known for dramatic dendrites is Rhyolite which is also a silica-based mineral. It's not a chalcedony because Rhyolite crystals are much larger than the microscopic ones that define chalcedony.  Some of my favorite dendritic rhyolite is found in the Sonora area of Mexico (and rock shows!)

And then there's dendritic sandstone. Sandstone doesn't have a place on the Mohs scale because it can be so soft you can rub it off with your finger up to something very hard that can be used in construction.

THE INSIDE STORY

From a mineral standpoint, dendrites consist of iron or manganese from nearby rocks that seep into cracks in the chalcedony and crystallize as they harden, creating the fern or tree patterns that make them so popular. In my readings about them, I was disappointed to see that they were nearly always described as black or brown in color, especially since I have two onyx rocks on my desk right now; one has dark blue dendrites and the other has dendrites the color of burgundy wine. It turns out that manganese oxide comes in a rainbow of colors including pink, violet, green and others.

 

NOT EXACTLY ONE OF THE ABOVE

Whenever people try to place walls around the natural processes, there are usually exceptions. This is a picture of a moss agate. Like other dendrites, it forms in the cracks of established chalcedony rocks. The difference is the mineral inclusion is green hornblende. It's as delightful to work as any of the others, and the startling green color is always an eye-catcher!


 

 

 


For all my readers in the USA, have a happy and blessed Thanksgiving.  As others begin to prepare for seasonal holidays, I wish you all the best.

Right now, I'm working on a blog post about how minerals get inside geodes--or when you see a channel between the interior of the geode and it's outside is the material expanding and flowing out, or is it the minerals lining the geode flowing in?

Your Lapidary Whisperer,

Donna Albrecht