Faceting Sphalerite!

Following a discussion on "PaulAhlstedt - YahooGroup" I have compiled this overview on "Faceting Sphalerite" with the comments by Paul Ahlstedt and me. You can se the entire thread here.

This is a summary of what you can find on Sphalerite in most mineral text books.
Chemistry	Sphalerite is a naturally occuring mineral made up of Zinc Sulphide. In nature there's invariably some amount of iron in the crystal, but also barium. Sphalerite is one of the base ores of mining. As a curiosum, Sphalerite with germanium is one of the main sources for this metal.
Hardness	Hardness is given as 3½ to 4.
Density	Density is between 3.9 and 4.2 with the average of 4.05. The higher the iron-content the higher density.
Refractive Index (RI)	Refractive Index 2.36 is almost as high as Diamond. Sphalerite's Dispersion though is extreme, 4 times as high as Diamond, at 0.16 Since Sphalerite is isometric, theres no birefringence. Anormal birefringence is rarely seen.
Crystallography	Sphalerite crystallizes in the cubic system. The most common form beeing the rhombic dodecahedron. Sphalerite has 6 very distinct cleavages parallel to the faces of the dodecahedron. Sphalerite is renowned for multiple complex twinning with crystals often showing penetration twins.
Color	Yellow, brown, red and black is common. Darkbrown to black is the normal. Green is rarely found. With higher iron content the color tends to the darker grayish brown and black. Barium gives a very nice yellow to red Sphalerite, at least with low iron.
Special	Sphalerite is triboluminescent - it gives off light when cut or crushed. Sphalerite with higher iron contents stinks when cut or crushed. The wellknown odour of rotten egg can be quite overwhelming. The same smell is a telltale in identifying Sphalerite, when you put a small drop of hydrochloric acid on the specimen. At least Iron-Sphalerites gives off hydrogen sulphide with it's characteristic smell.
Inclusions	Commonly included. Minerals like Pyrite, Chalcopyrite, Galena and even Silver are found.
Synthesis	Sphalerite has been made synthetic to study it's semiconducting abilities. Pure Zinc Sulphide is white, and synthetic Sphalerites are to.

Now! Let's turn to faceting!

Sphalerite to me has 4 - 5 factors giving concern in faceting.

1. Sphalerite's 6 cleavages seems to be all over.
2. Sphalerite's are rather soft.
3. Natural Sphalerite's are rather dark. Light color are of course found, but rarer.
4. The fourth factor - the smell - should be of lesser concern, since the lighter Sphalerites are low in iron, and hence smells less.
5. The twinning.

If you really want to cut a Sphalerite, the softness is a technical problem, not a great concern.
The color is as your rough gives it.
The smell when cutting is - well - as it is. You either stand up to it, or leave it alone.

This leaves us with the cleavages - and the twinning - as the main factor. Let's look at the rhombic dodecahedron a litle more.

At JCrystals they've made a Java-applet of a dodecahedron that's good to play with.
You may also build your own in paper or cardboard.
When you play around with the dodecahedron model it's important to realise the basic orientations of the 2-, 3- and 4-fold axis of the cubic system.

Rhombic Dodecahedron
1: The 2-fold axis	2: The 3-fold axis	3: The 4-fold axis	4: My preferred orientation	5: My second orientation

If you have a model in hand, Orient it in these 3 basic orientations, Remember the faces on the dodecahedron represents the cleavage and Think "Orienting it for faceting".

• You will see that rough oriented along the 2-fold axis leaves us with a cleavage in the table plane, which disqualifies this orientation.
• Orienting along the 3-fold axis leaves us with 6 cleavage planes on the girdle. Not good, but if you orient the girdle facets at skew to the cleavages, its feasible. The cleavages slopes at a very low angle - good because it's not likely to affect a facet.
• Orienting along the 4- fold axis leaves only 4 cleavage planes on the girdle. Thats better than before, but still you should orient the girdle facets at skew. The cleavages slopes away at 45 degrees - not good because that's our main measure of angles. But remember the high RI allows us to go as far down as 25.5 degrees, although the general recommendation is around 40-43 degrees.

1. I find that the best choice will be to orient the rough 10 - 20 degrees off the 4-fold axis in either up-left/down-right or down-left/up-right direction when oriented as in the fourth picture above. This leaves us with greatest chance of not hitting a cleavage plane with a facet. Some may come close though.
2. But orienting 10-20 degrees off the 2-fold axis may also be workable, although the any of the 6 cleavage planes around the table may be close to a girdle facet.
3. My third choice would be orienting along the 3-fold axis.

Now! How shall we find the cleavages? What we are looking for are the mirrorlike reflections from minute cleavages on the edges or rough surfaces of our stone. Anyone familiar with waterworn pebles of Topaz or cabochon cutting of Star Ruby will know the looks. When turning the stones, you will notice the reflections from the different cleavage planes. Mark them on your rough, and you should be able to find the 4-fold axis - 4 cleavages like in picture 3 above. When first you have located it, it's no matter to tilt it the right way, and mark the table. Then grind the table, dop up, and you are ready to facet.
Cleavage on a Waterworn Topaz Pebble	Parting on the Back of a Star Ruby
Photographing cleavages are rather dificult. You have to capture the reflection in the correct angle to show off the silky sheen so characteristic of rolled Topaz pebbles. On studying the Topaz pebble up close you may see the small cleavages as lines and minute mirrors in the surface of the pebble. The Topaz above is oriented almost vertical, so you are looking directly on to the clevage plane Partings are coarser since the phenomena is ascribed to muiltiple twinning, hence not as dominant and penetrating as cleavage but much more brittle.

Twinning With the twinning the problems with the cleavages multiplies. In one half of the twin the cleavage is oriented away from your cutting, in the other half it's almost inevitable that it's against. If your crystal is twinned, you may have to compromise on some facets. Maybe trade off polishing parts or all of a few facets. If you are lucky you find a direction to cut this specific facet, that polishes both halves of the twin. This is really time to play with reversing lap rotation and placing the gem on the lap. Keep a detailed log of your progress, or you will spoil the work done in prepolish polishing.

Twinning on the table of my Sphalerite

Cutting Sphalerite

I guess the real or most challenging problem with Sphalerite is really the scratching issue - that is where that gem will make a faceter out of you. It will often scratch, or get little tiny fragments break off a facet edge and skate across your work and.... well, you know what happens then.

Faceting should be on the wary side. Use as fine a mesh as possible - #600 or even less for cutting, a lot of water and not to high speed. On the other hand I did not find it difficult to cut Sphalerite - Calcite is worse. I polished on acrylic with diamond - #50.000.

Oh, and pay particular attention to - use SLOW lap speeds at all times. This IS NOT a material that can handle higher lap speeds.

Cutting log

This is one of those gems where you would wish you had learned yourself how to keep a log. I kept track of facets near on cleavages, to be aware next time I came around.
And thats necessary since you will go over each facet at least 2 times, and all the good work in cutting and prepolishing can be spoiled trying to polish for a fraction of a moment in the wrong direction to the grain of your gem.

But start off your log before that. Try to log down how you oriented the gem and which thoughts you had, doing that. Dopping method etc.

Log lap type, lap speed, where you put down the stone on the lap, if you cut it with reversed rotation, if you used more or less pressure, small strokes or long sweeps. Anything that might help you next time you go over that same facet.

And maybe the most valuable use of this log is for next time you cut a Sphalerite. This sounds maybe elaborate, and you soon cuts down on detail cutting the ordinary gems like quartz, but on these trouble makers like Sphalerite, Calcite, Spodumene, Zincite or Cuprite - the more details the better.

Dopping

You really shouldn't get Sphalerite hot during any stage of faceting, primarily this means NO wax dopping. Go with epoxy or cyanoacrylate, but remember that epoxy will shrink 5% or more, so keep the bond line as thin (and small) as possible - if you do not, you will exert considerable mechanical stress on the gem and possibly cleave it.

However, this are just warning notes as Sphalerite is a material like Fluorite where the cleavage is not as bad as would be thought (or bad as it is reported in other sources) - and most of you know that fluorite is really not that much of a bummer in terms of cleavage - feldspar is another, etc., but I am just saying best to be careful rather than sorry.

Laps and cutting

The greatest bit of wizardry is in choosing the correct lap - you should use the finest lap that you can to get the job done - start with #600, and get finer if you can. Paul Ahlstedt has often just cut Sphalerite with a #3,000 and let it be with it. Problem is, like Calcite, Sphalerite will give a sort of "suction" type of feel on the lap the finer the lap gets - and that will most often pull cleavage fragments out of the surface of the facet you are cutting if you are not careful.

For this reason, use light pressure, and using the mindset behind the cleavage models, always try to cut in a direction which compresses the cleavage planes - use your motor reverse switch heavily here for that. This may especially be very difficult to achieve on the table.

Paul Ahlstedt states that he has the best luck with lots of water on a Raytech NuBond in #3,000 for all grinding. In essence, you do the grinding and prepolishing all with one grit. This has the advantage that the lap is so fine and gentle, it really greatly minimizes the tendency to scratch and so on.
My #3000 NuBond imparts what I would say is about a #8000 finish, so it is actually pretty near a polish to begin with - subsequent polishing is then fairly easy.

I used #600 for roughing in, and #3000 on acrylic for prepolish.

The one problem here is that you can only use this method on normal jewelry sized stones - if you have one of those 100+ carat whoppers, you definately will have to default to the #600 for rough grinding.

Polishing

When you go to polishing - I guess the overall trick is to use light pressure, as fine laps as possible for the application, and very slow lap speed with lots of water - and the polishing method should produce as little friction as possible.

Use your favorite method with high lubricity, such as diamond or alox with extender fluid, light oil, etc, as this material will easily scratch and abrade with the slightest friction.
This is one gem you shouldn't use water & Linde slurry on, although you can if you are gentle with it (use a heavy slurry).

Paul recommend using the finest diamond that you can get away with, or you could even use alox powder b with a lubricant other than water and that would be fine as well - the issue here is that the more "glossy" you get the surface, the more that material kicks butt. :-)

Note however that the longer you stay on a Sphalerite facet polishing it, the more it will round - so get it as fine as you can with your prepolish and then hit it as quickly as possible to bring a good polish up and then move on to the next facet. The best polishing laps are those that are gentle. Plastic, and something like a resin Pol-A-Gem, Corian, Ultra-lap (and more traditionally a wax) should all probably be good gentle laps to use for polishing.

Also note that because of the softness, you can still OVERCUT while you are polishing - so pay careful attention that you keep your meets.

Clean up

A final note might be that sulfur and sulfur compounds are NOT good for your faceting machine, so be sure to clean up religiously after you are done.

A gem of some 10mm in diameter or greater in a lighter body color is worth the trouble, and a great addition to a collection - as well as being a great learning tool.

Design for Sphalerite

As for the design try "Sphaler-8" on the USFG website. It's designed especially for Sphalerite. But SRB's will also do well. I have used a step cut which displayed the caramel brown of my rough very well. Your choice of design should go for simpler designs to avoid facets closing in on the cleavages.

Step Cut Sphalerite

On the other hand the high RI gives you more room for playing with the pavillon angles. Thus darker material may safely be shallower, and still show reflectance. The dark color masks the dispersion, hence that's no concern. In very light material you might wish for a cut that displays the dispersion. Go for a higher crown, or even a BOG-optimisation of the design. But to get full advantage of the dispersion, make sure your facets are flat.

Aarhus, Denmark - October 27. 2005, Jón Olaf Svane with the aid and gross contributions by Paul Ahlstedt.