Back into tumbling (rocks)

 kw: hobbies, rockhounding, lapidary, rock tumbling, jasper

In March 2008 my brother and I went into the upper Mojave desert, to Lavic, to collect jasper. I wrote about the experience at the time; this post includes several pictures of the jasper. This is a view from the collecting area on Lavic Lake, north toward mountains above the Sidewinder Dunes, a hiking area. The colorful mountains are part of the same series of ranges that includes Calico, which is to the northwest, closer to Barstow.

Looking down near my feet from the same spot, small chunks of the unique jasper known as Lavic Jasper are scattered everywhere. Rock collectors call this "float". More than a dozen pieces of red and brown jasper are seen in this picture.

My wife and I had collected here a few years earlier, but brought back only about ten pounds of rocks. This time my brother and I gathered closer to 30 pounds. 

After I got home I bought a small rock tumbler—on this trip I had visited the Diamond Pacific store in Barstow and obtained grinding grits and polishing compound—and began to tumble in earnest. 

I wore out the cheap tumbler I had bought, so I did more research to get a better quality one. I bought a Thumler's Tumbler, and it has the staying power I need. I finished the last batch of really "clean" stones, those with no serious cracks or pits, in 2012, and reported on the that batch then (with more pictures of the lovely material). I replaced the drive belt once during those four years of on-and-off tumbling.

A couple of months ago I decided to go through the remaining rough material and pick out some that might be worth tumbling. I eventually settled on about six pounds of chunks and pieces, weighing between 10 and 120 grams each. It's best to tumble a range of sizes together. The small rocks get into indentations in the larger ones and help them get abraded more uniformly.

I put just over two pounds (967 grams) into the tumbler along with an appropriate amount of rough grit and water, and set it going. After six days the drive belt broke. I ordered a replacement along with some extra parts I thought I should have as spares, such as the lid, which can wear through. Here is a picture of the larger stones after six days of tumbling. At the start they all had sharp corners.

The third one from the left in the top row features in what comes next. I like it because of its mix of red and green sections. Green is the rarest color for jasper. This picture is one of a series taken November 30, 2023.

I had weighed the stones before I started (967 grams), and I weighed them at this point (925g). The 42 gram difference is 4.3%.

I reloaded the tumbler, including a charge of fresh grit. These clearly need a lot more material removed. I let it go two weeks and then took the stone out, to inspect and weigh (842g, down another 8.6%). I took another series of photos. The yellowish stone at upper left had such a hole in the end that I decided to break part of it off. I added the 2g piece so removed when I reloaded the tumbler and ran everything another two weeks. More photos and more weighing (742g, down a further 10.3%, for a total removal of 23.3%).

I'll quickly show three examples of tumbling progress. Firstly, the green-plus-red one I pointed out above.

I did my best to take all photos at the same scale. Here we see some interesting things happening. Firstly, of course, the stone is getting smaller. The white mineralized crack is still there, and at the right, white mineralization has begun to appear at the tip of the stone. The deep crack at the opposite end shows that this stone may not be suitable for further rough tumbling. I may need to use epoxy filler. Finally, the green material is seen to be rather thin, and much of it has been abraded away. This is a further indication that I should fill the cracks and proceed to finer grinding and polishing with this stone.

Secondly, one of the red stones.

This stone began substantially smaller than the prior one, weighing less than half as much. In the left and middle views, there is a deep gouge, which is almost totally gone in the third view. I'll probably remand this to another round of rough grinding to see if some of the whitish areas can be removed.

Thirdly, a greenish yellow stone with red striping, quite a small one. It started out as about ten grams.

The smallest stones lose the most proportional mass. This one has lost nearly all of the big gouge, so it is almost ready for finer grinding.

This batch of stones is presently set aside while I rough grind another batch. I noticed that the two sessions of two weeks' duration resulted in a slurry containing no trace of the coarse grit I started with. The six-day run still had plenty remaining (which I added back in). For the batch that is running now, I'll start with a ten-day run, and adjust run times to just barely use all the grit.

It will be several weeks before I have more to report. My plan is to do rough grinding on all six pounds of stones, then select which ones to continue with for finer grinding and polishing, or mix-and-match them into two batches for finishing.

This spring's stony harvest

kw: rocks, rock collecting, jasper, lapidary, photographs

A couple days ago I cleaned up the latest batch of rocks from my tumbler. These 32 stones are all Lavic Jasper that I collected in 2008. Their finished weight ranges from 88 grams down to just under 5 grams. I made closeups of five that I find particularly attractive.


This one looks the most like an agate, in close-up. Lavic Jasper is known and famed for having small fortification agates embedded in the matrix. This piece has a much larger fortification.


Here we have a breccia, that seems to have broken up when in a soft state, then re-cemented. The little bluish spots are some of the agate inclusions mentioned above.


This one has more of the blue agate showing. This feathery matrix is called plume jasper.


Much of the Lavic Jasper is dark brown, which is usually uninteresting. This piece, however, has a banded and swirly appearance that reminds me of the planet Jupiter's cloud bands. The piece is just 1.5x2 cm.


Here we have another piece that has a larger agate section. You can actually see into the stone in the dark vein across the middle.


All this variability shows why Lavic Jasper is my favorite semiprecious gemstone.

What I would cut if I were cutting

kw: rocks, minerals, lapidary

It has been a couple decades since I did any stone cutting. During the few years that I was a lapidary hobbyist, I soon found that I got the most satisfaction from cutting cabochons from precious opal.

Common opal is a whitish variety of hydrated silica (quartz plus water) with a very smooth texture. Precious opal, or fire opal, blazes with any or all colors of the spectrum. The difference is internal structure.

All opal is composed of spheres of silica, usually smaller than a micron in size, with a combination of amorphous silica and water in the spaces between the spheres. Two characteristics make common opal "common". Firstly, the spheres are larger and have a range of sizes. By "larger" I mean large enough that, no matter how they are packed, they will not diffract light efficiently; this typically means larger than one micron in average diameter. Secondly, the spheres, particularly because they are not uniform in size, are packed in a rather amorphous of chaotic arrangement. For this reason, common opal is rather soft, typically about 5.5 on the Moh's scale.

If the spheres are more uniform in size, they are likely to pack well into a closest-packing arrangement (like a well-stacked pyramid of oranges at the market). Their average size will then determine how much of the spectrum they diffract into the blazing colors of fire opal. Larger spheres diffract mainly red and yellow light, making red or orange fire opal. Smaller spheres diffract more green and blue light, while still diffracting some of the longer (redder) wavelengths. The smallest spheres produce mainly blue-green and green appearing stones.

When I was cutting opals, I found that they varied in durability. Not so much due to varying hardness, but because of how well the water content was bound in the stone. I learned to ask the dealer how stable the material was. An honest dealer will tell you, and point out which stones have to be kept in water to retain their fire, and which can be cut into a cabochon that will last the longest. Of course, the latter will cost more. Like a lot of rockhounds, I wound up with a jar of water containing nice looking opal rough sitting on my mantel, stones I'll never be able to cut, but they do look pretty.

During those years, I got to know one man who didn't own any lapidary equipment, but was cutting fine stones that won awards at rock shows. He said he'd buy a likely-looking piece of rough and grind it on the sidewalk next to his front step. If the fiery layer was buried too deep in the "common" portion of the stone, he'd pay the dealer to cut some of it off first with a diamond saw. At home he would simply hold the stone by hand and grind it on the wet pavement. He had one section of the pavement that was well worn from years of grinding, which was now his "fine grind" section. A light touch helped also, to achieve a nearly smooth finish. When he was satisfied, he'd go back to the dealer and pay to have the stone polished on a rouge-loaded felt wheel. The cost wasn't much, because a good polish took just a few minutes to produce.

Even the hardest opal is no harder than Moh's 6.5, so it cuts a lot easier than agate or jasper. I found I could make a finished opal cabochon in a quarter of the time it took me to finish a jasper "cab". If I ever return to the lapidary hobby, rather than collecting just those pre-finished items called "mineral specimens", opal will be first on my list of materials to cut.