Wednesday, June 5, 2019

Onyx or Not?

Green striped Onyx lapidary slab
When I was thinking about a topic for this post, I noticed a slab of onyx on my worktable. The pastel green layers were beautiful, and still are, but now I know it's not really onyx.

Darn.

It's still a lovely material, but it's one of those situations where it's a lot catchier to call a mineral Onyx, than Banded Calcite.

The banded calcite, CaCO3, is a carbon-based mineral that is commonly found in Mexico and India. From a lapidary standpoint, it's pretty soft at around 3 on the Mohs Scale. 




POPULAR DECORATIVE MATERIAL

Onyx bowl in browns






Like me, you may have a bowl, carving, or chess set made with this soft stone. The softness of this material also means when you start working it on your grinding wheels, it is best to start with your second wheel in so you don't take off too much material too quickly.








Mindat.org Onyx image




 TRUE ONXY is actually a variety of Agate, and as such, hits the Mohs scale between 6.5 and 7, making it more than twice as hard as banded calcite. According to Mindat.org, Onyx is a black and white banded agate or at times a monochromatic agate with light and dark bands. It does come in brown and red varieties; they are called Sardonyx. Mindat states, "Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved bands and onyx has parallel bands." The stones with parallel bands have been used to make cameos.


Now, bear with me.  In the Mindat.org image, I would have just called this banded agate even though the brown layers are somewhat translucent. They are parallel, but also curved.  Of course, translucence is not a requirement of Onyx. In fact, it seems to rule the material out.
Wikipedia Onyx image


Now, the more I get into this, the stranger it seems. Here's another image from Wikipedia they claim is onyx. 















Limestone Onxy
According to Mindat.org, there are a lot of stones called onyx, like this limestone onyx(CaCO3).

Checking around the internet, I found claims of green onyx (can be one of several items), onyx opal, rhodochrosite onyx and more. It seems as if any mineral that is translucent is being labeled onyx. 



GETTING REAL

Under the theory that "IT IS WHATEVER I SAY IT IS" I hereby declare that in America, Onyx is a translucent calcite. It is very soft. It has layers, most often parallel, and comes in a wide range of mostly pastel colors. The GIA might disagree with my declaration, but since they don't include Onyx as a gemstone in their online Gem Encyclopedia, well, my decision stands. 

WORKING THE SLAB



The slab at the top is the one that inspired me to do a blog on Onyx. So, of course, I had to work on some of it. 

I know the picture makes it look somewhat cylindrical, but it's not. I cut across the light bands and domed it, making the straight lines look like they are curved. From one side to the other, the back is flat. I played around with the top so that it is flattened in a way that mimics the curve in the rock making the picture a bit of an optical illusion. Then I put a bevel all around the back edge to keep the material from chipping.

Fun stuff!







I'd love to hear what you think on this topic. 

Until next time,

Your Lapidary Whisperer,

 Donna
 









Finely Banded Black & White Onyx

Sunday, May 5, 2019

CRAZY FOR CARNELIAN


 As you read this blog post on carnelian, you may think back to my recent post on garnets. I'm not focusing so much on brown/red/gold rocks as much as I am fascinated in the way they both handle light.

Carnelian is a 7 on the Mohs scale; garnets are about the same at 6.5 to 7.5, so you'd think that they'd be about the same to polish. Not always.

There is some variation within the scale. For example, the cab on the cotton candy slab just refused to take a real shine (it's photographed wet), yet the cab I made with some lower-value carnelian that is infused with what I've heard called "corn flakes", takes a great polish--except where the cornflakes surface.

Carnelian with "Corn Flake" inclusions
While I'm continuing to explore better ways to get this particular piece of carnelian to take a nice polish, I love working with it because of the stripes and the quartz on top.  It appears that at one time the quartz on top  had shiny crystals. Weathering damaged them to the point that the top was mushy-looking. They were, however, tight enough that when I ground them down a bit they make an interesting top with flashes.

Carnelian is something I'd decided to do a post on, and kept putting aside. For starters, the only carnelian I had was not all that exciting, except for one piece with quartz crystals on top. I've probably looked at it a million times. I'd love to take the "V" on the right side and make it like a heart with a crystal top, but the quartz crystals on the right are more weathered and smooshed than the ones on the other side of the dip. Any of you readers have ideas for me?


Then my husband and I took a weekend trip to Northern California and scheduled a stop at Chapman's Gem & Mineral Shop in Fortuna, CA. This place has so much rough, any lapidary artist would give their eye teeth to have it in their back yard. I'd give you a link, but they don't seem to have their own web site, everyone else is too eager to promote them.  In the yard, I came across a barrel that simply said, "Brazilian Agate". I was expecting the colorful, layered agate I've often seen at shows.

Nope, this huge space was full of carnelian. Now, carnelian and agate are chalcedony.When I first saw the sign, I expected the Brazilian agates that have layers of colors, but I was very happy to find this wonderful material.

It is believed that carnelian was originally named after the color of the kornel cherry. It comes in colors ranging from brownish red to orange and can vary from translucent to opaque. Like garnets, the specimens that are mined are often heat treated to bring out their best colors. Here in the U.S., carnelian is found in Oregon and New Jersey. Along with Brazil, where my latest rough was mined, it is found in Uruguay and India.


This carved Carnelian hook shows what a talented lapidary artist can do. It was fashioned by Stephan Roess artist, (stephan.roess@gmail.com).

This is a slab I cut from one of the pieces of rough I purchased at Chapman's. The fortifications where the agate bands suddenly change angle give it a lot of personality. I haven't worked it yet. It keeps smiling at me and as the Lapidary Whisperer, I listen to my stones and this one seems happy just the way it is!
Carnelian slab with Happy Face






Here's another slab quartz/carnelian with fortifications. It hasn't told me what it wants to be yet, but I have my eyes on it.


Until next time, I'm your Lapidary Whisperer,

Donna