Polishing Moonstone in a Rock Tumbler
Polishing Moonstone in a Rock Tumbler
Moonstone is a feldspar known for its soft, floating sheen. White and grey moonstone in particular can show a blue or silvery glow at the right angle. That makes it an attractive stone for the rock tumbler, but it requires more care than hard quartzes such as agate, jasper and rock crystal.
Moonstone is a feldspar known for its soft, floating sheen. White and grey moonstone in particular can show a blue or silvery glow at the right angle. That makes it an attractive stone for the rock tumbler, but it requires more care than hard quartzes such as agate, jasper and rock crystal.
What is moonstone?
Moonstone belongs to the feldspar group. The well-known light effect is called adularescence and is caused by light scattering in very fine internal layers. It is therefore not a colour layer on the surface. This also means that the visibility of the sheen depends greatly on the direction in which a piece has been cut or rounded.
Hardness
The hardness is around about 6 to 6.5. That says a lot about which stones you can safely combine.
Suitable for the rock tumbler?
Moonstone can be used in a rock tumbler, but it is not the easiest stone for beginners. Feldspar has distinct cleavage directions and can therefore chip or crack along natural weak points. A well-filled barrel, plenty of cushioning and regular checks are important.
Can moonstone go in a rock tumbler?
Moonstone can be used in a rock tumbler, but it is not the easiest stone for beginners. Feldspar has distinct cleavage directions and can therefore chip or crack along natural weak points. A well-filled barrel, plenty of cushioning and regular checks are important.
It is best to use moonstone in its own batch or together with other feldspars of similar hardness. Rather do not mix it with hard quartz, agate or jasper if you want to keep the risk of chips low.
- Preferably make a separate moonstone batch.
- Fill the barrel generously with stones and ceramic media.
- Check regularly for new chips or cleavage cracks.
- Assess the stone from different angles; the sheen can change a lot.
- Do not mix with much harder stones.
Polishing moonstone: step by step
Never work by time alone, but inspect the surface after each stage. A stone that still has sharp edges, deep pits or clear scratches is usually not ready for the next grit yet. You can also find the general process in our step-by-step stone polishing guide.
Select for strength and sheen
Choose pieces without deep cracks and look at where the best blue or silver sheen is visible.
Round off carefully
Use the coarse stage mainly to remove sharp edges. Check sooner than you would with agate.
Use enough ceramic media
Media help fill the batch and limit hard impacts.
Polish cleanly and calmly
Moonstone can polish beautifully, but not every piece will become mirror-smooth. Stop once the surface and sheen are good.
Important: clean the barrel, lid, rim, stones and media thoroughly before moving on to a finer grit stage. A few coarse grains left behind can cause new scratches in a later stage.
Which grit do you use?
For many tumbler stones, a progression from coarse silicon carbide to progressively finer grits works well, followed by a pre-polish and polish stage. The time required depends on the stone, the starting shape and how much material you want to remove. See the current range at grit and polishing media.
Coarse grit shapes the stone
The first step is intended for rounding sharp edges, removing weathered outer layers and grinding away minor unevenness.
Fine grit prepares for shine
The final grinding stages remove the scratches from the previous grit. Polishing compound only works really well once the surface is already completely smooth.
Which stones can you combine it with?
It is best to use moonstone in its own batch or together with other feldspars of similar hardness. Rather do not mix it with hard quartz, agate or jasper if you want to keep the risk of chips low.
Hardness is not the only criterion. Brittleness, cleavage, porosity and existing fractures are at least as important. Two stones can be about the same hardness and still react very differently in the same barrel. If in doubt, a separate batch is safer.
Common problems
| Problem | Likely cause and solution |
|---|---|
| Blue sheen seems to have disappeared | Rounding can change the best viewing angle. Turn the stone in the light before concluding that the sheen has gone. |
| Edges are chipping | This is often caused by impacts that are too hard or by existing cleavage planes. More filling and media can help. |
| Surface remains softly matte | Feldspar polishes differently from chalcedony. An extra-fine pre-polish stage and perfect cleaning can make a difference. |
How do you recognise good starting material?
The quality of the rough stone determines a large part of the final result. Preferably choose compact pieces without deep open cracks, loose rind or strongly grainy zones. Porous stones can trap grit and crumbly pieces can produce sharp fragments that scratch other stones.
Suitable
Solid, compact, no loose rind, no deep open fractures and a size that fits well in the barrel.
Less suitable
Very thin protrusions, porous zones, soft weathered skin and cracks that are almost open.
Polish stones yourself?
With a good rock tumbler, enough filling and suitable grit, you have the most control over the result. Especially in the coarse stage, take enough time: what is not solved there will usually still be visible later.
Frequently asked questions about moonstone
Is moonstone suitable for a rock tumbler?
Yes, but with more care than agate or jasper. A well-filled separate batch works safest.
How hard is moonstone?
Moonstone is usually around 6 to 6.5 on the Mohs scale.
Can moonstone be used with quartz?
Technically it can, but it is not ideal because quartz is harder and can damage the moonstone.
What causes the blue sheen?
The blue or silver glow is called adularescence and is caused by light scattering in fine internal layers.