Diamonds are the hardest. Yet …The reality is not ‘Diamond’, but diamonds – crystals with subtle structure felt during cutting. Often enough the effect of a subtle structure is not subtle at all. We have stories to tell
We managed to forcibly polish some small areas on this rounded rough but had no chance to further cut with conventional tools which existed 30 years ago. The white patches in the picture are glare, reflections off a couple of those small, flat polished areas.
There is some question of how a crystal such as this might have acquired its surprising shape. You do see playful, spherical forms of not a few minerals, including some gem species, yet, diamonds are their own world: the rounded rough is a Diamond which may have some unmanageable polycrystalline areas with marked grain boundaries, but it is still considered one coherent rough Diamond with plenty of irregularities. Such irregularities in diamond structure make the stone harder – harder to polish, for sure. (Strength does come from particular faults)
Some such tough diamonds cannot be cut at all; some might be worked to some extent.
The next picture is of a Diamond which was attempted to be cut around 30 years ago by my late father. Clearly noticeable on the table reflection are the signs of multiple and unsuccessful attempts at cutting and polishing a table facet:
I am counting at least six different (but very close) attempted polish angles (numbered in the picture). On the upper left side of the table are many miniature “knot” or “naat” inclusions (within the green circle), these are very small Diamonds included in the larger one, but neatly disconnected from it, growing in notably different directions relative to the large stone surrounding them, and impossible to gloss over. More signs of unwanted friction can be visible as miniature vertical lines (center, within the blue circle).
As luck would have it, you may compare these difficult areas with what a successful polish would look like at this stage: one small area of the diamond did take the polish well – it is now in a semi-smooth polished state (in the red circle, top right), with the surface wavering a little bit smoother than the others. The smooth polish stops at the border of a rough area (in the thicker, purple circle, above the red) containing a knot.
Diamonds such as this (below is another view of the same) are expected to contain zones with different grain directions which, in their simplest version, are called ‘macles’ (after a type of rough Diamond). This stone contains multiple macles.
When one macle’s grain direction is similar to that of the main Diamond, then the worst case scenario can be a simple raised area on the facet; this can be difficult, but not impossible to correct. Many macles with substantially different grain directions appear in groups make cutting impossible – as it may well be the case with the spherical stone in the first picture.
Diamond is the hardest. Yet… under great shock, applied along particular ‘grain’ directions, they would split. This property must have been understood very early, since the oldest technique for shaping diamonds, relies on it (‘cleaving’ – a story for another day). This property of diamonds is certainly not obvious: how could one tell how diamonds break! ( & not for lack of trying to tell – www).
For once, in some of the most beautiful stones, clean fault lines open, even if the roughs are not being ‘cleaved’ along their grain, but cut after other directions.
Some beautiful and rare rough such as the famously vivid orangey yellow Diamonds from Zimmi, a small town in Pujehun District, Sierra Leone – are known to develop feather inclusions during the various cutting and polishing stages… The price for holding COLOUR !
On account of this reputation of Zimmi, I had planned the Diamond in the next picture to be cut without any sawing (dividing) the crystal. Even so, after grinding the top to form the table facet and the four crown facets, numerous feathers developed. The crackle goes deep into the stone, as a gridwork of great fractures reflecting light the wrong way; this is no longer a transparent diamond…
This is Grand Failure of a diamond – nothing subtle, yet the details took several shots to show properly: their depth, their reaching the surface, and the beautiful colour of the stone – each view required different lighting for the camera to ‘see’ (what is in a picture ?).
The next shot shows the table reflection of said Diamond: looking closely, you will notice the open surface marks showing how the feathers have developed out of one unfortunate site. Bear in mind that this Diamond was free of inclusions while still untouched:
The next picture of the same diamond shows the colour and the depth of the surface-breaking ‘feathers’. Light reflecting off them will no longer play as it should against the cut facets; if the cut could be completed, it would not ‘work’ – a pity for such a great and rare color…
Laser cutting the table instead ? Yes… it fails in the same way:
The diamond in the next picture was sawn (divided in two) using a laser – hence the tell-tale plow-like marks on the surface:
In the next image, a horizontal fine line is noticeable – the mark of a deep feather reaching the surface:
The depth of the feather is made visible in transmitted light:
These stones split not because of the stress of cutting, but because the cut removed parts of the outer layer of crystals that had contained internal tension.
By comparison, correct cleaving along clean planes that cross diamonds end to end, is not as interesting:
This is what it looks like:
The causes for such accidents can be subtle discontinuities in the structure of the crystal, that allow splits to become directed along the grain (not unlike the cleaving tools mentioned above did)… Such ‘subtleties’ could be inclusions reaching from the surface to surface, boundaries between different ‘macles’, feathers that met the wheel at an unhappy angle, or just the gesture of laying the diamond on a 3600 rpm velocity wheel, for reasons unknowable.
These two (above & below) are cleaved diamond, so to speak, not that the placement of the cleavage plane did anyone any good:
Imagine shaping diamonds – only – by splitting them like this ! (give or take means for slight polish, taking more labour than anything else). Yet, this was done – achieving the earliest diamond cut models. The facets resulting from cleaving respond to the structure of crystals, not to anything else you & I might will. A lost art!
Cleaving is such a lost art that finding reading materials presently is rare. I am still a lucky one as cleaving was my first apprenticeship in the entrance into the diamond cutting arena.
Too much to be said …
by Yoram Finkelstein