FMF - Friends of Minerals Forum, discussion and message board

[Rei]

What do you all recommend for polishing stones, esp. slabs?

The technique I tried was first cutting with a diamond cutting disc on a grinder (full suit, mask, goggles, gloves... don't want to lose an eye or get silicosis!), then grinding down with a sanding disc (or on hard material, with the edge of a diamond disc), and then using a buffing attachment for the grinder with a buffing compound.

Cutting: Mixed. It worked fine on non-chalcedony, but chalcedony had a tendancy to fracture when cutting, it was like cutting glass. On my test piece, when I was done, the largest remaining piece was about 40% the size of the original, the next largest maybe 20%, and so on down from there.

Grinding down: Worked fine on all samples. Though I was still apprehensive with the chalcedony given the fracture risk (it didn't break, but I was concerned), and it took a long time because I didn't want to use much pressure... every now and then the wheel would grip a little bit on the rock (due to variation in hardness) and I'd get worried.

Buffing: Poorly. Clearly I need some kind of step between grinding and buffing, because the marks from grinding are all still clearly present on the stones. To make matters worse, the buffing compound tended to fill the grinding grooves and didn't always want to come out. Also, the buffing pads tended to rip up really quickly on the edges of the rocks. Maybe I should just forget about "power" buffing altogether and stick with hand sanding?

I'm including pics below. Tips/recommendations?

[dironot]

I am curious as well as I have been using a dremel for sanding and polishing small pieces and have been less than happy with not only its ability to even out grooves but also the standard polishing compound is reddish and soaks into the stones I am polishing.

[rocks2dust]

Once you've ground to shape (or flat), you need to step to progressively finer grits to get rid of the grooves and scratches. It takes a while, but you can do this with a combination of diamond files and finish off using wet automotive sandpaper to 1500 grit or finer (only works for stones with a hardness of less than Mohs 8 or so). A final buff with a polishing compound on a felt bob should finish. Some people start out by doing the whole series by hand.

The crazing is caused by heat buildup. Using wet grinding and sanding is preferable, but pausing frequently to dab a bit of water on the stone works for some materials. There is a series of videos using a Dremel on youtube:

https://www.youtube.com/watch?v=7Tio1tBwbfg