I try to get all or most of the pits and open fractures removed and the front and back smoothed to produce a more acceptable slab for you to finish for jewelry or specimens.
To get a beautiful shine, my recommendation for finishing our slabs is to start with 600 diamond grit, if 600 doesn't remove all grit lines on the slab work your way back and use (320, or 240) and then 600 to remove all grit lines, sometimes you may need to finish with 1000 grit before polishing. Follow the instructions in the next step.
This is what I do when I grind a slab, with 320 to 240 laps, on last pass over the slab I turn the slab and grind it in the obosite direction, if I look at the slab with a power scope and I see lines crossing each other I know it's not smooth and I have to continue with the last grit until I don't see grits crossing each other.
You need to remove all grit lines before you use your polish in order to get the best shine. The smoother you can get the slab the brighter it's going to look. Also use a light touch when using 320-600 grit laps.
Sometimes it's not possible to smooth out all the grit lines, but you will end up with the best possible shine when you follow my guidelines.
A lot of material may only get a bright luster shine and not a mirror shine. This is generally caused by the makeup of the slab and not the softness. If you remove all the grit lines before polishing the luster shine will still make a beautiful slab.
TYPE OF POLISH AND LEATHER PAD TO USE
There are a lot of different polishes that can produce a mirror finish and a lot of different pads (leather skin) that will work with a particular polish. It's been a long time since I changed my pad but I think it was Elk skin. I know it took some time to break in the skin before it worked really well, also use the smooth side of the skin. Elk skin is an expensive leather. I use 3m Feathering Adhesive to glue it to a disc. When it's time to change it, it will make it easier to remove.
After years of experimenting with polish I settled on 61 Rapid polish. 61 Rapid Polish is also used for tumbling or hand polishing. I'm sure it has a mix of material in it's makeup, like Aluminum Oxide and Linde A, maybe even Cerium. The polish is sold by the lb on the internet. You just add water to a small amount and make a slurry to coat the leather, don't get the pad to wet, a little is best.
I experimented with a lot of leather skins with polish and I found that Elk skin works best for me. I also use the smooth side of the leather. I've always had to break in new leather by polishing a few cabs before it started producing a mirror polish in a shorter time span, don't quit until you get that mirror polish. Most stones will get a mirror polish but you will find some material can only produce a luster polish. I use the small rubber type paint brushes to put the polish on the pad.
Watch out for sharp stones or polishing the edge of slabs, the pad can tear if you're not careful especially after a long time using it (years) it wears thin. If the slurry dries out too much on the pad the stone may overheat, especially with opal type of material. As the polish tends to dry out on the pad add a little more to stop the pulling on the stone. You will need to Dop your stone so it's not pulled out of your hand. Only make enough polish that you will use, or add water to it after you are finished and store it in an air proof container.