Star sapphire advice needed

Among a jar of stuff that dad has had for decades, I found a good-sized star sapphire and decided to have a go at it, my first attempt at a star sapphire.

Made sure it was oriented correctly to show the star. Encountered numerous very fine cracks as I worked it down but all the bigger ones I manged to get rid of. It now displays quite a nice big, even star with straight legs that covers pretty much the whole surface of the cab.

One problem - it seems un-polishable. The crown has polished up quite nicely but the sides of the cab simply refuse to polish. In fact, they refuse to even pre-polish and once I reach #600 (resin-bonded diamond), no amount of time spent working seems to have so much as the slightest effect. So the top is highly polished while the sides retain the look of having been rubbed with fine sandpaper.

Can anyone suggest anything I could try?

Lefty,

If diamond isn't working for you have you tried a very felt/chamois or vinyl disk with optical grade cerium oxide or tin oxide run at a very fast speed? This has worked for me when polishing bombs!

Thanks Dughug.

I just tried tin oxide on a soft lap running as fast as it goes. Polished it for about 5 minutes but the result is the same - no discernable effect

Maybe #280 to #600 is too big a jump, even with diamond? Problem is, I don't have anything in between. Not resin-bonded anyway. I do have a #360 diamond flat lap, not sure if I should very carefully touch it to it to see what happens.

Lefty

sapphire to polish on either Tin Oxide or cerium oxide needs to be really fast as a lot of the polishing effect is generated by the heat created and not so much by fine abrasion.

What you tried with a 280 to 600 jump should have been do-able followed up with finer diamond for the polish.

As a facetor do you charge your own laps? If so, try 360 diamond powder mixed with a medium of sorbelene or Vaseline or baby oil etc. then treat the stone like you are skin polishing it - i.e. hard felt or wooden tips with a dremmel like rotary tool. Wooden bamboo BBQ skewers work well (cut off into 2 inch pieces). Then progress 1200, 8000, to 50,000 using the same method.

I'd leave the flat lap well alone though.

Good luck with your attempts on the star.

It's a kind of fine skin-peeling effect - I assume it's the same phenomenon that I've heard facetors complain of when they talk about sapphire facet peel.

#600 has almost no effect at removing it and the next grit down simply increases it.

No idea how to attack this problem.

Lefty,

I see by your post on ALF you gave the flat lap a burl - & you said it scratched like crazy - trial & error is one way to find out what does & doesn't work.

I presume you have the bonded belts on an expanding drum which may account for the orange peel effect as when you "push" the stone in to cut the rubberised drum tends to form around the stone and cuts unevenly- particularly where the sapphire crystal has grown.

You could try a hard disk with wet & dry sandpaper, say around 300 or so to take out the orange peel effect if you have a buggared bonded belt try cutting it up & using it on the disk this would stop the deforming of the belt and give an even cutting surface with perhaps a lesser chance of the softer areas cutting quicker.

Yep, they're resin-bonded diamond wheels Dug ( Cabking 6v3 machine).

Have been doing some looking up and found a site dedicated to star sapphires. They reckon that star sapphires should not be cabbed using any bonded diamond grits (either metal or resin) above #600 otherwise they have a tendency to an orange peeling effect. They apparently only use loose grits from that size up. Actually seems a bit counter-intuitive because in my short experience, it's loose grits that seem to have a greater tendency to undercut things. Guess sapphires must be an exception.

Not sure I have enough star sapphires to justify buying a bunch of different loose diamond grits in sizes I already have bonded to cabbing wheels and laps. But unlike the only other one I've done which was just a total bomb, this one seems much better and displays a nice,big even star.

Might just put her aside for awhile.

Should you ever be near Redcliffe, bring this stone and drop into the lapidary club there. They have 3 flat laps that get charged with diamond powder (come on a tuesday, when they get freshly charged) 400, 600 and 1200 grit on a hard calico back. Worth a try I'd say.

Thanks Piep

There was originally a cabbing lap with the old Hall faceting machine I inherited but it wasn't in the box when I got it. Dad and I turned his shed upside-down yesterday but to no avail. Might have to buy a cabbing lap or two.