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[ileney]

Despite the foot of snow and hazardous conditions, 600 people showed up yesterday at the Maine gem and mineral show. I waited until the second day, today, with blue skies and clear roads, and by the opening time at 10:00 am, the parking lot was entirely full except for three or four spaces! Soon after, it was a madhouse.

Along with facet and cabbing grade clear and rose quartz, cabbing grade amethyst, a slab of bubbly clear chalcedony, and a few other things, I scored a small but heavy cobalt stone (below) and a box of star rubies (picture also below), something I know nothing about. (I like opals, but didn't buy any. There wasn't much opal there aside from a couple of boulder opals and small welos, doublets, a really lovely spencer triplet, plus cabbed lab grown opal (not synthetic) that looked odd in a way I couldn't put my finger on and was prone to fracturing.) 


1)The "cobalt" is pink, not blue. Why? It is very heavy. Can it be cabbed or is it poisonous? It was $3 so I won't be upset if it is just good as a specimen.
2) Are the "umba" star sapphires decent quality? The vendor was an older couple who appeared to be destashing and those were the only corundum they had. They they threw those in in return for me buying the facet/cabbing grade stuff.
3) Will the corundum kill my Pixie wheels? How do I cut them? I tried reading how to cut these but people use technical jargon about the c axis, which means nothing to me. Most of these stones are hexagonal. In the middle of many of the hexagons is a raised triangle or a series of raised triangles falling sideways. To cut them correctly, am I correct in assuming that with the stone laid flat so it looks like a hexagon,  the center of the top triangle should be the top center of the stone and I should slope this as much as possible down the sides of the hexagon to make the hexagon into a oval and to make the stone as tall as possible? My assumption is that with the triangle on the top in the middle, I should get a star?

Thanks for any information on these.

[rocks2dust]

I think what you have is a chunk with cobaltite (cobalt sulfide arsenide). The pink is probably erythrite. I was just playing with a cabochon of skutterudite (another cobalt arsenide) that also had some pink, but its pink was cobalto-calcite and looked different than yours (larger patches of very pale pink). You probably shouldn't ingest or breathe any of it while grinding (I haven't come across any health warnings regarding working with it), but otherwise it should be safe to wear. Cobalt is still used as an ingredient in pigment, so should be safe as a gem. The arsenic content should also be bound up enough to prevent toxicity from wearing (similar to other arsenic-bearing stones that are used for gems).

[Ranger_Dave]

I have a big chunk of that corundum. It fluoresces cherry red under short wave. I haven't thought about cutting it.

[lithicbeads]

Corundum is very hard on sanding wheels  and the stone that results from relatively low grade corundum is seldom worth the effort. The best luck that I have had is finishing the sanding on a  set of flat sanding discs with no rubber pad underneath  the disc.When I cut lots of harder than quartz stones I came to rely on  a well worn 600 grit steel wheel that eliminates the worst scratches before you sand  but it has to be used with a very fine touch as it does not conform to the stone as a sanding  soft wheel does and can leave lines that can be time consuming to sand out. In the olden days I have no doubt that  600 grit sic belts on an expando was the key to success followed by diamond on wood or phenolic laps.

[gemfeller]

I've cut quite a few star corundums (rubies/sapphire) and can tell you that diamond wheels will hold up *much* better than the old carborundum wheels I started with.  It was a question of which was ground away more and faster, -- the corundum or the carborundum (about the same hardness).  I sanded and polished them on maple laps with diamond compound.

You should be able to grind and rough shape the stones quite easily with your hard diamond wheels.  You can use 220 or 280 Novas to do final shaping.  After that I'd recommend a Star Cup from Graves or a Star Lap and diamond compounds to do the final sanding/polishing.

 http://www.gravescompany.com/CabMate.html

I think the Star Cup is exclusive to Graves but the Star Lap by Crystalite is available from many suppliers including Graves.

The tops of stars in corundum (if there's enough rutile "silk" to create them) are always aligned with the C-axis of the crystal.  It's not that complicated, check the image:


C-Axis.jpg (2.67 kB . 172x130 - viewed 212 times)

I've never cut star corundum from that particular source and can't guess what your results will be but good luck!  I'm happy to answer more questions if I can as you go along.

One final tip.  You can use several things to check if asterism is present: STP, honey, any high viscosity oil or a commercial preparation called Star Refractol (available from most supply houses). Put one small drop in the center of the crystal's C-Axis (that would be the tops of the crystals in your image).  It will create a tiny "dome."  Observe it under strong light.  If enough silk is present to create a star you'll see a tiny star shape inside it. 

[ileney]

Wow! As usual, gemfeller, your information is so clear and helpful. Thank you so much! I am going to give one a try and see what happens. I have stop and honey handy so I'll give those a try first before I order anything new. Again, many thanks.