Polishing Black Onyx

[Dihusky]

A client has given me a job with Black Onyx, a pair of cuff link inserts around 12 x 8mm. Cutting has posed no problems apart from the propensity to chip on the facet edges and the black gunk left on the Laps, but polishing... that's a whole new and very frustrating story. This material simply won't polish no matter what I try!

Has anyone had any experience with Black Onyx and how did you polish it? This material seems to shed the dye/treatment and it acts like a barrier.

My last resort at this stage is going to be trying the Cerium polishing lap I use for opal, or the same optic grade Cerium with a Dremmel.

Any ideas or guidance would be appreciated

[Rusted]

Black Onyx? Do you know what it really is as Onyx seems to cover a lot of mystery stones.
Is it the Calcite/Travertine Onyx or Agate Onyx or something else? It's not Jet is it? Black gunk on laps, maybe Psilomelane.

I just cut a small black stone for a replacement in a ring, a tiny curved thing in three dimensions a real cow to cut, anyway I got a small bit of Epoxy on the surface of my stone and used Acetone to clean it off, and found out the other original stone which was still in the ring on the other side dissolved with Acetone, It only marked the surface slightly but it was a real problem to repolish. I don't know what it was, plastic, rubber, bakelite or what.

[Dihusky]

True Black Onyx is rare and expensive so most is grey chalcedony then treated either by soaking in cobalt chlorate and chlorammonium or by prolonged immersion in sugar syrup then heated in sulphuric acid to carbonise the sugar.

The stuff stinks like hot tar when cutting on coarse laps which would I feel point to the sugar treatment and explain the black gunk. SG is 2.6 so in the correct range for onyx, beyond that the brain cells are past caring, just want to get the damn stuff finished with a polish the client is happy with.

Horrible stuff to work with!

[Rusted]

If its Chalcedony then Tin Oxide or Cerium on a lucite should do.

[Dihusky]

I've never had much success with cerium on Lucite, always seem to go backwards and end up with badly scratched facets, which is why I normally polish everything with Diamond, but discovered Diamond don't work on this stuff!

Don't have any Tin oxide unfortunately so will give the Cerium a try on the felt lap and keep my fingers crossed.

So far lesson learnt with this stuff... Don't cut it!!!

[Rusted]

A good solution for things that are difficult to polish is draughtsmans film. get it from Office shops.
It looks like tracing paper but is some sort of synthetic and it is tough, cut disks to suit your lap put a drop of oil on your lap to stick it down. Use whatever polish you want on it, commonly Cerium or tin. It works a treat and if it gets contaminated just lift it off and replace it.

[Dihusky]

Cerium on the opal pad did the trick