Crystal size vs finish

​No two pieces of nephrite are the same.  Most have a range of crystal sizes, and many have the variations across even small sections of stone.  Only the best quality stone has an even, cryptocrystalline (very, very fine) texture throughout, and with no veins or fractures through it.
 
That’s why some pieces of stone are more expensive than others.  It’s generally the case that you get what you pay for.
 
A piece of nephrite with coarse, or variable crystal sizes will never give a mirror finish across the whole piece, but you will be able to polish any finer crystalline areas to a high sheen.  The variation can give character to a piece!

​Figure 1 shows a (mostly) coarsely crystalline piece of South Westland stone.  You can see the individual crystals in the areas reflecting the light, as well as the variations in crystal sizes.  You can also see the variations in translucency:  the greener areas are good, the areas with white flecks are intermediate and the brown areas are poor.

​There are often clues found on the exterior of a piece of jade as to its interior texture and structure – you might see a vein for example.  But let’s just stop here for a moment.  What’s the difference between a vein and a fracture?  A fracture is a crack in the rock.  I’m not going to talk about “open” (unfilled) fractures here - you must plan your workpieces to avoid them, though superglue can sometimes help to stabilise a worrying, small fracture in late-stage finishing.

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Figure 2 shows the outside of a  piece of Cowell.  Maybe it is possible to imagine some indistinct, relict bedding planes sub-parallel with the cut face (top)?  There is evidence of a small vertical fracture near the lefthand end of the top edge.  The righthand edge shows some changes to the mineral structure due to weathering.  What you can't see is evidence of coarse crystals in this face.

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Figure 3 shows a different side of the same piece of Cowell.  This is a more weathered face, and you can certainly see the open fracture running up the middle from the right face of the large "V" at the bottom of the picture, as well as another running across the top of the other face of that "V".  The rounding of the face attests to not only weathering but also erosion.

Figure 4 again shows the same piece of Cowell.  It is a cut and polished face, and you can clearly see the fracture running up from the "V" in Figure 3, as well as a number of smaller fractures off it, and mostly radial fractures cutting the bottom and left edges of the rock.  There is also one on the right which closely follows the broken edge.  You can see the weathering rim on the left and at the bottom, the effects of which gradually dissipate into the rock.  There is a faint dark band down the middle of the face reflecting a possible line of flow during formation of the jade.  And there is evidence of wavy, sub-parallel banding in the right half quarter of the face which is probably relict sedimentary structure - the flecks themselves are the result of a mineral in the rock breaking down through time.

​Fractures occur:

• during cooling and shrinkage of the jade and it’s host rock after formation
• in earth movements some thousands or even hundreds of millions of years later
• from weathering of the rock at or near the surface over hundreds of years
• in the mining operation, which inevitably involves breaking out and “man-handling” a piece of rock.  If explosives are used they produce millions of fractures throughout the rock and render it only useful for road-stone and maybe beads

 
Some of these fractures fill with mineral-rich fluids in the ground, which deposit the mineral salts they carry and close the fracture – and those become the veins I’m talking about here.  So, the mineralogy of the vein filling may be completely different to that of the jade, depending on the temperature and pressure of the rock and what minerals were mobilised by groundwater or geothermal fluids at the time.  As a result they may be harder or softer than the host jade.
 
So let’s get back to the piece of fictitious rough you are looking at.  You may also see several sets of sub-parallel fractures defining the sides of the piece.  These come-and-go, merge with other planes of fracture and/or define changes in the structure of the rock – possible hangovers from the original lithology of the piece before re/crystallisation began.  You might see depositional features like traces of ancient algae or different crystal sizes representing different sized sedimentary grains, and even reflecting the different lithologies of the original sedimentary rock.

Figure 5 shows the beautiful face of a piece of polished British Columbian nephrite (picked up in Kirk Makepeace's yard, origin unknown).  You can still see colour variations and even banding running through the piece, but neither affected the crystal size or hardness.

​To see more subtle variations in crystal size you probably need to cut and slab your piece of rock.  But to really find out the quality of the stone you may have to polish a face.  If the crystal size is even across the face, you are onto a winner.  It certainly makes carving easier.  If there are more highly polished, raised veins you have a piece of rock with a bit of a history.  Or there might be small, irregular areas protruding or sunken into the face by just a few microns after you’ve polished it, reflecting different hardnesses and chemical compositions for the two parts - the softer areas are undercut preferentially to the harder parts.  It’s not the end of the world – just be aware of it when you design your piece or plan the finish. 
 
According to Len Gale in his book “Greenstone Carving, A skillbase of techniques and concepts” (published by Reed Publishing (NZ Ltd), pendants were historically only given a matt finish by Maori carvers, and there has been a renewed trend to leaving faces or entire pieces with a satin or matt finish over the last 20 years in NZ.  I’ve found that leaving one face rough on a simple pendant creates interest and people snap them up.
 
If however you want a mirror finish on the piece you need to be aware of the hardness variations in your rock and choose not just the right piece of stone for your work but the right part of the right piece.

​Figure 6 shows a piece of coarsely crystalline jade which has a much finer and harder crystalline vein running through it, lower right to upper left, and which was a conduit for fluids during formation of the rock.  There is also a slightly softer, dark mineral (possibly hornblende?) which developed in clumps in the rock. 

​Of course, you are most likely to only discover the variations once you get down to the final pre-polish stage of, say, 1000 grit.  Life can be like that!
 
There are ways of dealing with undercutting of the softer material, like preferentially and carefully grinding the higher, harder areas, and then re-finishing the piece with a minimum of work.  Or you can also rub it all down again and leave it with a satin finish which will hide the softer material, which often doesn’t take such a high polish.
 
And one last point.  A piece of semi-nephrite (a piece of stone not completely recrystallized as nephrite) will be likely to have parts of the original, precursor, rock remaining.  These will have a different mineralogy to the areas which have melted and re/crystallized.  They may pose difficulties, and may break more easily than the rest of the stone, so it will be worth using this rock for making chunkier pieces.  Turn it to your advantage!
 
So the next time you buy some jade, you need to look at not just the colouring, but every aspect of the rock, as the outside carries clues to what’s inside, and also how you should cut it, but that’s a different subject entirely!