Hi again, Tony,
In addition to the polishes I mentioned in my earlier reply, it also occurs to me that moving from 100 grit right to 500 might be too steep. My usual grit sequence -hardly original; used by many others- is: 80, 220, 280, 600, 1200, 3000, then whatever hi-grit diamond polish works, usually 15,000 and up.
Also check your water flow over whatever grinding surface you’re using. If you’re using a Diamond Pacific Genie, with just that little bottom spritzer thing to recycle the same water onto the bottoms (!) of the wheels, in my experience residue can build up on the wheel and cause scratches and tear-out: the 2 sets of 3 wheels are sharing water, so your nice 500-grit wheel can pick up larger pieces of debris from the 100-grit wheel. And the whole wheel does not get rinsed. Whereas a design similar to a CabKing with its copious overhead water flow -- or any grinder with an add-on fresh water supply like Hi-Tech’s All-U-Need, to flood the wheel -- should rinse away contaminants AND keep the wheel cooler.
If you’re using a flat lap, are your laps really, really clean?
And finally, it could be the material itself. Jet only has a Mohs hardness of 2.5 to 4 – that’s not very hard!
Some stones have natural pits. Or possibly whoever slabbed it should have cut the rough 90 degrees around from your surface plane. If this is the case, you should be able to get a good polish on an edge: with 100 grit, regrind one edge to a steep edge and step through the rest of the grits to see how well the edge polishes.
If the stone turns out to have natural pits, you could try stabilizing it with Opticon and redo the polishing. (Of course if you then sell the stone, the right approach is to disclose to the client that it has been stabilized.)
It’s hard to tell from photos whether that’s the dreaded orange-peel effect. If it is, it usually happens later, in polishing, not in grinding. But if those are scratches or pits, you should be able to see them with the naked eye. Do you have a 10x loupe to examine the surface? There are books full of theories about polishing: eg, slow speeds vs faster speeds, so experimenting might be your best bet.
Hi, I grind and polish a variety of stones. I have two types of Jet, one is somewhat soft, the other is very hard. It's the hard one I'm having problems with. The softer Jet polishes to a mirror finish. The Hard is very matte like, it's very hard to "see" the Jet.
Grind with diamond: 100 (shaping), 500 (more shaping a polish front and back, 800 (pre-polish), 1200 (polish). I do have a 3,000 K lap.
Sandpaper: 1000, 1500.
Luxi Polish: Blue then White
Diamond Polish on Felt pad: 14,000 then 50,000 k.
(I have Googled, searched archives and forums, no luck)
Pic: One on the left is soft, hard is on the right.
(Been a member of Ganoksin since 1994, best on the net for info on stones and jewelry)
Hope you can give me some advice.