Magnesite for Rock Tumbling and Cabbing
This image shows a 5-pound bag of magnesite rough, dumped into a colander and sprayed with water to reveal its full color. Click on the image for a larger view. This rough shapes quickly in a tumbler and produces beautiful white tumbled stones with gray webbing
Magnesite Tumbling and Cabbing Rough
Size: mostly 3/4" to 2" pieces
White, Cryptocrystalline, Silicified
This magnesite is perfect for rock tumbling because it has a very fine grain size and is slightly silicified. The silicification makes it a bit harder than pure magnesite, and the very fine grain size enables it to be tumbled into smoothly rounded stones without cleaving. Pure magnesite has a Mohs hardness of 3.5 to 4.5. This material has a hardness of 5+. That allows it to be easily polished in a rock tumbler using TXP aluminum oxide polish or most other popular rock polishes.
Here are some pieces of our rough magnesite that were tumbled using the Rock Tumbling Recipe below.
INTERMEDIATE Rock Tumbling Recipe:
Silicified Magnesite (3/4 to 2 inches)
When tumbling this magnesite two things should be considered:
1) Shapes Quickly: This material has a Mohs hardness of 5+. That means it will require a little less time in the first grinding step than the agate, jasper, and quartz materials that most people tumble.
2) Media Needed: This material is in pieces that range between 3/4" and 2" in size. For that reason, ceramic media should be added to fill the voids between the pieces of rough. These will provide a smoother tumbling action and deliver grit to the surfaces of the larger pieces of magnesite. We would tumble a mixture of about 25% media (or small rough) and 75% magnesite. Additional media should be added after the first tumbling step if the barrel volume drops below recommended levels.
Recipe for Rotary Tumbling
Barrel slurry dumped down the drain will harden like concrete.
Coarse Grit Step: Fill your barrel 2/3 full of rough and media (or small rough). About 20% to 30% of the material in the barrel should be ceramic media or small size rough. Then add 2 tablespoons of coarse (60/90) grit per pound of material in the barrel (rough + media). Add enough water to cover the rocks and tumble for one week.
Medium Grit Step: Put your cleaned rough and media back into the barrel. If the barrel is less than 2/3 full, add additional broken-in media to get the barrel up to its proper operating level. Then add 2 tablespoons of medium (150/220) grit per pound of material in the barrel. Finally, add enough water to put the water level just below the top of the rocks. Then tumble for 1 week.
Fine Grit Step: After clean-up, put the rocks and media back into the barrel with 2 tablespoons of fine grit per pound of material and enough water to bring the water level up to just a little below the top of the rocks. Tumble for one week. Your rocks should be really smooth after this step.
Polish Step: Make sure that your barrel and rough/media mix have been cleaned thoroughly of all grit and rock mud. Then place the rough and media in the barrel with two tablespoons of TXP polish for each pound of material in the barrel. Then add enough water to almost cover the rocks. Tumble for one week.
Burnish If Needed: If you have done a good job of tumbling, this tumbled magnesite will have a nice soft glow. It will not have the "glassy" polished luster that you can produce on agate. Burnishing will probably not be needed, but if you want to clean up the rocks, you can find the full burnishing instructions here.
Howlite and Magnesite Are Often Confused
Magnesite and howlite can have very similar appearances. They are so similar that they are two of the most commonly confused and misidentified materials in the lapidary trade - but they are completely different minerals. Howlite is a calcium borosilicate hydroxide with a chemical composition of Ca2B5SiO9(OH)5. Magnesite is a magnesium carbonate with a chemical composition of MgCO3.
You can confirm that this is magnesite because, as a carbonate mineral, it is very slightly soluble in dilute hydrochloric acid. If you produce a small amount of powdered material, apply one drop of dilute (5%) HCl, and watch closely with a hand lens, you will see a few bubbles of effervescence. Howlite should not effervesce - unless it is contaminated with a carbonate mineral.
We went to the extra step of confirming the material's refractive index. We obtained a spot refractive index of approximately 1.52 to 1.70 with birefringence blink. This is consistent with magnesite. Howlite's spot refractive index is approximately 1.59 and it never exhibits birefringence blink.
Fine Grain Size Eliminates the Cleavage Problem
Magnesite sometimes has perfect rhombohedral cleavage - just like calcite. That type of material would be extremely difficult to tumble into nice polished stones. However, when the crystal size is small enough the cleavage does not present a problem in a rock tumbler. The small crystal size limits the length of a cleavage break to the width of a very tiny grain. This is why stone carvers who work with marble select the finest grained marble that they can find. It's why Michelangelo chose the Carrara Marble to sculpt the Pietà, the only sculpture that he ever signed.
Often Dyed As a "Turquoise" Imitation
Silicified magnesite and howlite have both been used to imitate turquoise. They both readily accept dye. A few decades ago a few enterprising people dyed these materials a nice turquoise blue color, then placed them gift and tourist shops without identification information. When this was first done lots of people purchased them, thinking that they were turquoise. Today, many people know that dyed howlite and dyed magnesite are just one of many types of fake turquoise. However, some people are still be fooled.
In addition to faking turquoise, howlite and magnesite are dyed almost any color you can think of. Red, green, yellow, orange, purple, pink and more. So, if you see a colored rock with gray markings, it might be dyed magnesite or dyed howlite.
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