Here is my method of sanding and polishing slabs and geode halves in just a few minutes. It involves sanding with a dry Silicon Carbide 8" by 3" belt on an Expando drum.
Here is a photo of the slab that I will demonstrate on. It is a Brazilian Agate and the saw cut very roughly and left rather significant saw marks. It is a slab that I got last Spring at the local rock pow wow.

Here is a photo of the belt. It is a 100 grit that has moderate wear. When you feel of the grit surface you can feel the grits rather easily but they are not sharp.

Here is another view. There is some buildup of rock dust on its surface.

Because the sanding is done dry you must use a high quality dust mask. I use these N95 rated masks that can be purchased at most hardware or paint stores.

Here is a photo of the begining of the sanding.

I always sand only in one direction, parallel to the long direction of the slab. I sand a few strokes (four or five) and feel of the slab. If it has gotten hot enough that you can't comfortably touch it , stop and wipe it with a damp cloth to cool it down. Don't wet the belt because it will wash the rock dust out of the belt. Your objective is to get a buildup of dust on the belt. Stop and look for progress in removing the saw scratches. Here is a photo after 3 or 4 minutes of sanding.

Another couple minutes of sanding.

A few more minutes. We are getting real close to being finished with the sanding.
There are a few scratches left.


Here we are finally finished sanding. I look for no diagonal saw marks (one of the reasons I always sand in only one direction, preferably across the saw marks) and for very shallow fine sanding marks.

Turn it a couple directions to check for the sanding marks.


Here is how the snding belt looks after the sanding process. More dust than when I started.

The time elapsed so far is 12 minutes including picture taking.
Next I go to my 12" diameter, carpet covered wheel with Cerium Oxide paste.

Here is a photo 15 minutes later before I washed it (and dropped it in the sink, breaking off the tip of the unsanded part of the slab).

Another unwashed view. You can see the polish streaks still on the top right corner.

Two views of the finished slab after it was washed off.


I intentionally left one end of the slab in its original condition as a comparison.
If you are working a thicker slab (this one was 3/16" thick) or a geode half you can do the sanding steps faster because there is more mass to the piece and it won't heat up as fast or require more time to cool the slab when it over heats.
Bob