While viewing the front of the Brazilian agate slab, I spotted a much more interesting group of features that I could use to enhance the overall aesthetic of the piece. A three-inch circle would allow me to bring out all of the best patterns that the piece could offer. I sketched the circle and added a curved feature on the top so I could drill a hole for suspending the piece as a pendant.

Where do I get my project inspiration? First, this column pushes me out of my usual cab shape routine and into using different materials, shapes, textures, patterns or colors. I read a lot of books on lapidary and jewelry design and also subscribe to a couple of magazines that have current jewelry styles and materials such as blue gems and minerals. Sorting through my multiple boxes of slabs often triggers my imagination too.

MAKING THE SHAPE

Before and during grinding, I make sure to follow proper lapidary safety precautions. While grinding the piece to a preformed shape, I used a one-inch diameter Mizzy silicon carbide wheel run dry to shape the inside curves on the top section. I drew a line halfway up the girdle as a guide for shaping the dome of the cab. I used an ultra-fine Sharpie felt tip pen to draw the line.

After I finished shaping and sanding the front of the Brazilian agate cab, I turned it over and focused on the translucent area. It was to be enhanced and ultimately be the main focus of the cab. From the start, I intended to drill four holes in the back of the cab so they would show through from the front. I drew a curved line in the center of the translucent area so I could lay out the position of the four holes. (It gives more visual interest if the holes aren’t in a straight line and if they are varied in size from larger to smaller.) I also made sure that they were evenly spaced between the holes.

DRILLING & FINISHING

I drilled small pilot holes for each bit and placed the bits in these holes to verify the correct spacing. I started drilling with a small coarse diamond bit. To get the best drilling action on the bits, I turned them on their sides. If you try drilling by placing the bit vertically, the very center of the bit does no work because it is spinning in one spot. By placing it sideways, the diamonds in the bit sweep across the cab and remove material.

I gradually stepped up in bit size and grit size until I reached the desired hole size. Because I wanted to ensure that the holes were highly visible from the front I drilled the holes up to 1mm from the front of the cab. I used an inside caliper to measure the 1mm depth. The final grinding step involved using a very large bit to chamfer the top edge of the hole to remove any chips.

The finishing steps were done with shaped wood bits using 220 and 400-grit tumbling media followed by shaped wood bits and cerium oxide polish.

The finished Brazilian agate piece came out much better than I had expected!

This story about Brazilian agate appeared in Rock & Gem magazine. Click here to subscribe. Story by Bob Rush.

Step By Step

The post Brazilian Agate Inspiration first appeared on Rock & Gem Magazine.

The colors and their patterns are generally prevalent throughout the piece, so it isn’t difficult to orient these colors and shape the cab to retain the best pattern.

Rough Material Selection

I decided to make a Brazilian agate piece with a center stone of rose quartz and three accent stones made with Ethiopian opal. Though the center stone was made from a piece of asteriated rose quartz from Auburn, California, the slice that I had wasn’t thick enough to make a sphere that I could utilize in determining where the star was oriented.

Previously I have worked with Ethiopian opal to make cabs for rings. Typically, the material has some fractures, so selecting a fracture-free rough piece with a general dome shape can take some time. By being selective in choosing the rough piece for color, shape, and defects you can get the best yield. Most of the rough has a coating of sandstone and some sandstone inclusions, so you need a strong backlight to highlight the amount of penetration of the sandstone. The light will clearly reveal the depth and location of any fractures within the rough. Most dealers have a strong light to evaluate the rough.

After flattening the bottom of the rough, I prepared the dop sticks. Because the cabs will be rather small at 6 mm across, I used lengths of wood skewer sticks for dop sticks. I further reduced the diameter on the end of the sticks by shaping them with a file. I applied a small piece of dop wax on the stick by holding it over an alcohol lamp to melt and shape the wax. I carefully heated the opal piece by holding it in my fingers up to the side of the flame. Concurrently I heated the dop wax until it started to get very soft. I brought the wax up to the opal and stuck them together, and I reheated both taking care not to apply too much heat to the opal.

Trust Templates

The next step was to grind the perimeter until the stone was round. I frequently checked the shape with a template. After it was rounded, I shaped the top to a dome shape.

Next came the changes. One of the characteristics of this opal is that it exhibits a condition called hydrophane. In the distant past, the hydrophane effect meant that when the opal dried out the color disappeared, thus it must always be kept in water to see the colors. In the case of Ethiopian opal, the hydrophane effect has the opposite results. As it gets wet while being worked, the color slowly starts fading until the stone approaches near clarity.

This can be frightening if you are not aware of the opal losing its color. The color does restore itself after a few hours of being dry. Sellers of this opal always keep it dry where sellers of most other opals usually keep the rough wet so the colors show better.

Retaining Original Color

While working my stones, I had varying experiences with the colors. The stone that started with the brightest color had the least color loss while working it. The next brightest had a total loss of color. The final stone had a completely different change. It started out with only a green flash of color and it turned white during the cabbing process. I hadn’t experienced this effect before. In comparing them together on the dop stick, these differences became evident.

Fortunately, all of the stones returned to their initial colors, though the green flash stone took considerably longer – about 36 hours – to get to that restored condition. With the restored colors the mounted stones look quite attractive.

This What to Cut column about Ethiopian opal previously appeared in Rock & Gem magazine. Click here to subscribe! Story by Bob Rush.

The post Bench Tips: Working With Ethiopian Opal first appeared on Rock & Gem Magazine.

After cleaning the sludge from my 18” saw I got back to slabbing some older pieces of Brazilian Agate that had been in my pile of rocks since I moved into the Gold Country about 10 years ago. I noticed a 7” by 5” Brazilian Agate that I had cut a face onto before I moved. The face looked a bit dull, but there was a black pattern that piqued my curiosity. I mounted it in my saw and started slabbing.

After I got the second slab off the rock, I compared it with the first slab and there was an interesting difference. The slab that had been exposed to the bright sun and summer heat for many years had a reddish-brown layer that differed from all the remaining slabs. The layer is a definite color change, not just dirt or debris on the surface. I’m going to experiment with some heat treating on a piece of this material in the near future.

Intriguing Patterns

The pattern on the slabs is mostly black on a translucent milky background. As each slab came out of the saw, I got rather excited to see the pattern in each slice get more and more interesting. On the third slice, an intriguing pattern presented itself. I kept slabbing to see if the pattern would get more intricate, but it just kept changing into a larger pattern not as suitable for the particular project that I had in mind.

I sketched the pattern that I wanted the cab to be and trimmed and ground to this pattern. I liked the vague and indistinct look of the pattern, but when I flipped the cab over the other side spoke to me as being deeper and more foreboding as it stared back at me. I chose to finish this side as the front of the cab.

I ground the concave edges into the cab with my 1” diameter dry grinding Mizzy wheels. Using the same wheels, I tapered the sides into the front surface. Sanding was accomplished on my 8” silicon carbide belts on an expando drum. In my process, I move the belt off the side of the wheel about ¼” and push the concave part of the cab into the belt. The belt flexes and conforms to the cab and the sanding goes quickly.

Seeking Feedback

The polishing step on the concave edge is accomplished on the rounded portion of my carpet-covered polishing wheel.

I’m really pleased with the outcome of my work, but I am struggling with whether to mount a stone in the “eye” of the cab or not. I’m interested in hearing from some of the readers about their thoughts on this matter.

The post Bench Tips: Following the Pattern first appeared on Rock & Gem Magazine.

After I showed my wife the first cab I completed in which I added details to the back, she asked when I was going to fit colored pieces of stone into the circular features on the back. I hadn’t thought about it until then, so I decided to give it a try for this month’s project.

After some searching, I found a suitable piece of Brazilian agate that had a dark lower pattern and a translucent upper area. The top layers had a thin, black pattern that was very close to the surface and limited my ability to get much of a dome on the front.

Creative Use for Back of a Cab

What I had in mind was to shape a small hemisphere of carnelian agate that would closely match the size and shape of the hole that I was going to make in the back. I had the concept that the carnelian would show through, much as the sun shines through the back of clouds.

I made sure that I could match the size of the diamond ball bur to the size and shape of the carnelian hemisphere. I started by shaping a disk from a thick slab of carnelian from which to make the hemisphere.

I made sure that the disk was thick enough—at least half the diameter of the original disk—to make a hemisphere that would match the depth of the hole in the back of the cab.

Utilizing A Sphere Cup

On my lapidary grinding unit, I roughed out the preform for the hemisphere. I took care to make the shape as round as I could so I wouldn’t have to spend much time finishing the shape in my small sphere cup. I found a cup that would allow me to get the right size for the hemisphere.

With the sphere cup installed in my flat lap, I used 220 grit and 400 grit to shape and sand the hemisphere. I didn’t have to polish it because the Epoxy 330 needs a slightly rough surface to get the best bond.

With the hemisphere shaped, I started grinding the size and depth of the hole with a slightly smaller diamond ball bur. It is much easier to enlarge the hole to fit the hemisphere than the other way around. This step went very quickly, and I got a near-perfect fit between the carnelian hemisphere and the hole in the cab. I mixed some Epoxy 330, glued the pieces together and left the assembly under my bench lamp to set up.

Unexpected Outcomes

My final steps were to finish shaping the dome on the front and polish the back.

Unfortunately, the color from the carnelian didn’t project through to the front as I had expected, even though I had drilled the hole to within 1 mm of the front surface. Apparently, the translucent area of the cab wasn’t as clear as I had perceived it to be.

What turned out better than I had expected was the fit and color of the carnelian, as seen from the back. In my next project, I’ll be to explore fitting more and different colors of hemispheres into various cabochon materials.

The post Bench Tips: Working Cabs from the Back (Part III) first appeared on Rock & Gem Magazine.