17 mm square, 55 carat cerussite faceted by Duncan Miller from rough provided by Rockey Ollewagen

Cerussite is lead carbonate (PbCO3) and probably the best crystals come from Tsumeb. These can be large and glassy, usually clear, but sometimes grey, brown or red. It has a hardness of 3½; a specific gravity of 6,5; distinct cleavage in two directions; is very brittle and extremely heat sensitive. The refractive index is high, at 1,90 to 2,07; and the birefringence very strong. The dispersion is high. The high refractive index and dispersion should make it a spectacular gemstone and it occurs in fairly large crystals. So why don’t we see more faceted cerussite? The rest of the list of properties makes it quite daunting – lack of hardness, two directions of cleavage and extreme heat sensitivity.

Recently Rockey Ollewagen gave me a rough crystal, a bit brownish and battered, and said “See what you can do with it”. I like a faceting challenge, so I did. First I looked for cleavage cracks and ground a flat angled well away from them to act as a temporary table facet, using a 600 mesh sintered bronze diamond lap. Then I prepared a flat dop with a layer of wax, flattened against a cold face-plate dop in a transfer jig. When the wax had cooled I glued the stone to the wax surface with a small drop of cyanoacrylate ‘superglue’. Then it was a matter of grinding away surplus material with the 600 mesh lap until I had a roughly square preform for the chosen design, “Squartuguese” by Marco Voltolini (available for download on GemologyOnline). It is one of my favourite designs – a corner-cut square with a brilliant pavilion. The pavilion facets were cut with a 1200 mesh sintered bronze lap and polished with no problem on a wax lap (generously donated by Rob Smith of African Gems and Minerals) using Linde A aluminium oxide in quite a thick paste. So far so good. Now for transfer.

This presented a problem because obviously I could not use hot wax in a cone dop. So I tried gluing the pavilion into the dop with superglue. It didn’t set overnight because the fit was not tight. So I filled a cone dop with a two-part epoxy putty, pushed the pavilion of the stone into it to make a negative impression, pulled it out again, and waited to for the putty to set, overnight again. It did. So then I glued the pavilion into its impression in the putty with superglue. When that had set, which was quickly, I heated the initial dop to soften the wax layer. Ting! went the stone!! Aargh!!! A very obvious crack appeared across one corner, threatening to spall off the entire corner; but it didn’t. So I scraped and dissolved off the residual wax and positioned the new dop in the faceting machine’s quill. As I started cutting I was puzzled. Where had the corner crack gone? All that was left of it was a tiny trace near one edge and no amount of neck twisting and illumination fiddling could get it to reflect. I had to accept the improbable, that it self-healed. Never happened before. Unlikely to happen again. Anyway, I cut the crown on the 1200 mesh lap, modifying it slightly to give it a bit more height, to preserve material and enhance the dispersion. Polishing went without a hitch, including the table. Then, to remove the stone.

A soak overnight in a jar of acetone turned the superglue/putty combination into a jelly. The stone literally fell out, but left a chunk of pavilion behind. Not the corner, but a chunk in the middle of the cone. I suspect that the epoxy mould had distorted slightly as it cured and that when I pushed the pavilion too firmly into the mould it caused a chip. After a few days I decided to recut the pavilion. The extremely high refractive index meant that the pavilion facets could be lowered by a few degrees to remove the shallow hole left by the chip. So I re-dopped the crown, using a thicker layer of wax with superglue, and a clean cone dop to reposition the stone on axis in the transfer jig. With nice big girdle facets on the stone, re-orientating the dop in the quill was easy, and recutting the pavilion went smoothly. Removing the stone from the dop also went without mishap, despite the existence of a cleavage crack that opened up in the middle of a side facet at some stage. Anyway, the stone held and the final result was worth the effort. The photograph (courtesy of Jo Wicht) picks up the inclusions more strongly than the naked eye does, but also shows the coloured dispersion spectra very well.

Grinding cerussite made a horrible, goey, toxic, white slurry. I am not sure I want to do it again. But if I do, I will grind two opposing flats on the rough and cut the crown first. This will enable me to transfer dop to another flat face, using a drop of superglue on a flat wax surface and avoiding the use of a cone dop altogether. I have no experience with cold dopping using epoxy glue, but that could be an alternative.