FMF - Friends of Minerals Forum, discussion and message board

[John S. White]

Would someone care to explain what is meant by a fibrous diamond? I have a hard time imagining such a condition in diamond. Are you sure that you are not referring to the skin of the diamond, which to me appears to be drusy? Never having examined one of the cuboidal diamonds under high magnefication, my guess is that the drusy surface is probably composed of tiny pyramids, which are octahedra.

[Pete Modreski]

Well, I'm assuming that the bulk material of those diamonds, in their outer portions, has kind of a fibrous, outward-radiating texture, just like chalcedony is fibrous quartz.

I guess one would need to go actually read some of these papers that have been cited here, to see what all they really say, & whether they have any actual pictures, SEM or other images...

[Pete Modreski]

Hi Jim, back to the diamond crystals! I've received several more emails via the MSA listsesrv, but this one from Bob Downs, of the U. of Ariz., has a link to a photo of a really spectacular and large (I should say, awesomely huge) cubic (or better to say, cuboid or more-or-less-cube-shaped) diamond crystal!
Pete

Here is a jpg image of one of the more famous diamond cubes, courtesy
of Mike Scott. The diamond is about an inch on edge. The cut stone is
a 1 carat piece for size comparision. This should give an idea of the
upper end of the range in sizes, and of quality.
Bob

https://www.geo.arizona.edu/xtal/group/images/diamondcube.jpg

[Jim]

Hi Pete,

Yes, that is the specimen to which I was referring earlier. It is amazing! It was the cover of MR a few years back for their gold and diamonds issue.

That said, I have not seen many references to gem-quality cubic diamonds of any size. I have located a few very small examples, but nothing that even approaches the Scott specimen.

Thanks!

[Pete Modreski]

Ah, yes, I thought I'd seen it in that issue, but I hadn't gone back and looked. It's magnificent, and a magnificent photograph, too.

P.S., one of my favorite specimens of diamond seen in Tucson, was this (well, there are two of them in this photo), displayed, if the digital date saved with the image is correct, in 2003. Renate Schumacher took & sent this picture to me, because I hadn't taken one--it's a little blurry, but conveys the idea. I was impressed by this polycrystalline-looking, semi-round diamond, reminds me of a lopsided golf ball; and with it, a sphere polished out of a diamond crystal, which is also kind of unique. I guess this would be considered a "bort" diamond. I don't know what person or institution these specimens belonged to (perhaps I should look & see if they are pictured in that Diamond & Gold issue); but, here they are.

[Elise]

Hi Pete,

I ran across this old thread while searching for something by Professor Sunagawa - I love that photo!. Thanks to Dr. Tetsuo Irifune (Geodynamics Research Center , Ehime University, Japan), I had the opportunity a few years ago to examine a sphere polished out of synthetic nano-polycrystalline diamond. One of the papers I wrote about the experience is in part a literature overview, including the cutting and polishing of a sphere and the structure of natural polycrystalline diamond - it may be of interest given this discussion here. All the G&G journal's papers are now available in open-access pdf form as well as hardcopy: Skalwold, E.A. (2012) Nano-polycrystalline diamond sphere: a gemologist's perspective. Gems & Gemology, Vol. 48, No. 2, pages 128-131.
https://www.gia.edu/gems-gemology/summer-2012-diamond-sphere-skalwold
(link normalized by FMF)

Cheers!
Elise

[John Betts]

You can see a 71.64 carat black cubic diamond if you search Google for "71.64 carat dark-gray cubic crystal"