Polishing Obsidian: An Insight from MildKyle
By Kyle V. Richcreek
World of Rock Hounds
Let’s talk about polishing rocks and fire obsidian – a two for one tale.
The lapidary arts is a unique hobby in which you can take a stone in its rough state and through various stages of tools and grit, turn it into something beautiful.
It can be as simple as a straightforward cut and polish or as in depth as carving an animal from the stone to bring it to life. There is virtually no limit to what you can do with stone.
For those that practice and apply the lapidary arts, each artist will have their own set of skills and knowledge in specific fields of the trade.
My specialty has been obsidian. It started with rainbow obsidian and over time, transitioned into fire obsidian and a fire obsession.
The Back Story
I once took a lapidary class where I took a small stone, cut it in half and only polished one side while leaving the other half of the stone unpolished to have a “before and after” memento of my first worked stone. After I worked on this stone, I wanted to learn how to polish obsidian because it’s both somewhat hard and brittle while being very difficult to get a good polish with minimal scratches.
It all started with the purchase of electric blue rainbow obsidian that I paid a shop $20 to cut and polish. They had reached out and said that they tried multiple times and could not achieve a polish on the stone and handed it over. That stone would set me on my quest to do what they could not.
I grabbed a piece of rainbow from the small collection I had at the time, brought it to the shop where I had taken my class and got to work. The initial idea was to use it as a test piece but it ended up being a work of art.
Over time, I had made wonderful pieces, but then I discovered fire obsidian. The rarest and most expensive variety and oh did I really want some.
I managed to acquire two slabs for $20 each. I polished them both and brought them back to the shop where I acquired them and the response I got was, “I should have never sold you that”.
On one of those slabs, I had cut an imperfection off and turned it into my first fire obsidian necklace. This is where I observed how much of a steady hand and an eye for detail is needed to work the material because all it takes is too long on a sanding stage and that color is gone. Forever.
Obsidian can be found in various places in the world, but fire obsidian is a naturally rare variety of volcanic glass that only comes out of central Oregon. Even though obsidian gets its black color from magnetite, the layers of fire obsidian consist of concentrated nano-crystalized magnetite layers which the thickness is that of a wavelength of light. Which means, if you have a piece with a single layer and you make a cab out of it, you can see straight through that layer with no issues. The colors in fire obsidian can vary from colorless (silver) to every color of the rainbow on a single layer.
It’s also important to note that the layers of fire obsidian can be inconsistent whereas layers of rainbow obsidian are typically more consistent and even on a horizontal plain.
Finding That Fire
It’s both an easy and a complicated process. Sometimes there are visible lines on the crust of the obsidian which could be a good indicator that there are multiple layers of the stone but you don’t know if those layers are black in color, ribboned layers, or even possibly sheened layers.
It helps to have an exposed corner of 45 degrees or fewer so you can see inside the stone. I found it helpful to have a black container of clean water and a flashlight to submerge the obsidian into and then use the light to see what it holds. It helps if the obsidian is clean and again, has a visible window into the stone.
The other way is to just start cutting the stone. For rainbow obsidian, it’s somewhere around 40 to 45 angle cut across the layers for the best way to expose those colors. Since the layers are consistent, it makes it easy to cut.
Fire obsidian, however, can be tricky since the layers are not always uniform and can at times be all over the place and can even cut into other layers of the material, making it incredibly difficult to expose the fire later.
Once you do expose that fire layer, it becomes a matter of slowly and carefully grinding away the waste material that sits just above the layer because the closer to the layer you get, the brighter it can be.
You do have to think ahead and keep in mind how close you are to that layer after each stage.
It’s similar to that of working a color bar in precious opal – it takes a steady hand and lots of patience.
To start with: to work a stone is mostly the same across the board.
It doesn’t matter if your tools are flat disks, wheels, motorized or non motorized equipment, diamond/sanding bits for a Dremel, or tumblers. Everything goes through many stages of progressively higher grit – you will eventually get a polish.
A few things To keep in mind when you are getting into the hobby:
1. Understand the material you are working with (softness, hardness, fractures, toxicity, etc.).
2. Smaller surface areas work differently than larger surface areas.
3. Know your equipment and how the various grit stages will turn out – You can use “test” or scrap material for that.
4. Practice safety and use proper PPE because you do not want rock dust in your lungs.
Today’s topic is obsidian, which is both somewhat hard with a mohs hardness of 5.5 but also super brittle and prone to scratching. The early stages of working the material are very similar to working any other stone material but since it’s prone to scratching it requires a bit more patience and an eye for detail.
Grinding a Stone (In General)
Always use water when using any kind of diamond tools to not only keep down on dust but to also help prolong the life of the tool. Because, well, diamond wheels aren’t cheap!
The stages I use are 80 grit and 180 grit hard diamond wheels for shaping of a stone. Typically starting at 80 grit if the piece has many high spots and is very uneven. However, if the stone that I’m working on is relatively smooth-ish, I will start at the 180 grit diamond wheel to shape it further.
Be mindful of this stage and keep the stone moving while you are working on the wheel so you can avoid any facets forming (flat spots with “sharp” edges) but if that’s your thing – then go crazy!
Sanding a Stone (In General)
The following stages are basically a rinse and repeat; if you want 6 facets on a large stone, you will have to keep track and do every step on all those facets. It can be a lengthy process but it can also have a beautiful result.
The stages that I use will depend on the project and the machine.
I have two cabbing machines that have the following grit stages for the soft sanding wheels before I take it to polish: 220, 600, 1200, and 3000.
My bigger machine for bigger projects has two expandable drums where I can easily swap out the wet/dry sanding belts where I use grits before taking it to polish: 100, 400, and 600.
For that machine, I do not throw away belts unless they have a visible tear on them because using a worn belt can sometimes act as a higher grit stage which can be helpful for some projects.
When sanding a stone, I find it important to work in a singular direction to avoid a mesh pattern at polish. Being sure to keep using water and keep the stone always moving when sanding to avoid unwanted facets on the surface you are working.
Each sanding stage is essentially removing the visible marks from the previous stage by removing material and leaving its own scratch marks on the surface.
The higher in the grit stage you go, the smoother it will get and will continue to remove any visible scratches from the previous stage.
It helps to have a dry rag and a good light source to continue to check your work by drying the surface of the stone before proceeding onto the next grit stage. With enough practice you will see any spots/scratches that need to be worked out before proceeding further.
With obsidian, the 280/600 or 400/600 grit stage is going to be the stage you are going to focus the most time on in order to remove any scratches before going to 1200 grit.
On the 1200 grit stage, take your time to remove any visible scratches from the lower grit stage.
After this stage, you should have a nice semi-polish shine to it. If you don’t see any scratches, you can either take it to a 3000 grit wheel or take it straight to polish.
Time, patience, and an eye for detail are the biggest things for the sanding stages.
What I use is a Berber carpet polishing pad with cerium oxide and it has done a fine job over the years that I’ve been using it.
The cerium oxide needs to have a nice slurry before using. It needs to be the consistency of toothpaste if not just slightly runnier. If it’s the consistency of tomato soup, or any kind of soup, then too much water was added.
I turn on the machine, spritz it with water until the carpet is damp; apply the cerium to the wheel until I can see the carpet turn color; spritz a couple more times for good measure, and then get to work.
My machine rotates counterclockwise and I always work on the left hand side of the wheel due to personal preference.
I always lead with the bottom edge and never with the top to avoid the stone catching on the wheel and being sent into oblivion. Adding pressure is key and with this step: the stone orientation to the working surface does not matter, only pressure and keeping it moving!
Check your work every so often with a rag to help wipe away the polish.
If, at any point, you check your stone and there is a streak where there is no polish on the surface, it means that the polishing wheel is drying up and more water needs to be applied.
Go back to stone size, the bigger the surface area, the longer it will take, and the smaller the surface area, the less time it will take.
The polishing stage is where any blemishes will appear if time on a sanding stage was not sufficient enough. In my experience, as frustrating as it can be, it will be okay. Just go back to 600 grit and start over.
Working stone is more forgiving than working wood.
If you would like to know about polishing obsidian, please feel free to check out my YouTube channel as I have several videos on the subject and I demonstrate the steps that I use. It’s free to watch and free to subscribe.
A big thank you to Jared for this opportunity to share some of my knowledge regarding lapidary on his awesome website!
Thank you for joining my Ted Talk.