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[Aaron]

Hi,

New to the forum and excited I have found a place to talk opal (my fav gemstone). I am wondering if anyone has any tips to prevent pitting of the iron in andamooka and boulder opals? I head slow is a good starting point, but what speed is actually slow (6,000rpm, 3,000rpm...)? What final (or intermediate) polishing compounds would you suggest?

Thanks!

[lithicbeads]

 I have cut a huge amount ( hundreds of pounds) of Oregon opal over the years and  we traditionally used a bull wheel with cerium for polishing after working the stone on all diamond.The rhyolite matrix on that opal is not prone to pitting. For the final prepolish we used a variable speed machine  and inspected closely and very often. I liked the slower speed as it did less cutting and more sanding on the edge transitions on opal which can be oversanded easily.My favorite trick with any matrix problem was to use a diamond 600 grit hardwheel. It will cut both the opal and the matrix at the same rate  and is gentle on the brittle matrix stone. Since it cuts evenly there is much less oversanding of the opal relative to the matrix and less deformation of the stone in those transition areas. I now love flat  sanding pads for the final diamond sanding steps. For quick touch ups of problem areas you can use a pad ( well worn in all cases with opal ) with no rubber backing  and for the overal sanding use a rubber backed pad. I use these flat pads in conjunction with diamond sanding wheels on a cab machine.They complement each other and with the use of the 600hard wheel make a precise and speedy combination. I prefer all diamond polishing for smaller opals now but when the stone  gets over about 50 grams  the oxide polish is much more efficient.

[vitzitziltecpatl]

Have never been misled by Lithickbeads' tips. You could print that post and laminate it and get great results.

Speeds? Most common motor rpm is 1725, but for opal there are times you'll want a variable speed to go slower.

You mention Andamooka and Boulder. If you're referring to Andamooka matrix (usually "cooked" to get black color) there are some differences between that type and typical ironstone boulder opal.

Here's a link to tips from a miner/dealer of boulder opal. Think he definitely qualifies as an expert on that type.

http://www.koroit.com/koroit.com/Cutting_Boulder_Opal.html

Happy Cutting!

[Aaron]

Thanks both! Really appreciate all the details. Yes, the andamooka is matrix. Have some really nice rainbow rough and want to make sure I do it justice.

My cab machine has variable speed, but the range is from 0-100, so not sure exactly where this should be set, any ideas? On the flat laps, do you advise at any point to use a sponge backing for smoothing things out, or does the rubber do the same? And is the diamond vs tin oxide a preference, or an additional step?

Thanks for all your help.

[Phishisgroovin]

i usually go through many many many steps of laps & worn belts up to a diamond paste 14000 and 50000 to get my polish on opals.
Avoiding heat on my stone (even matrix created opals).
I heated a $50 rough opal when i first started lapidary not knowing anything. tried sticking it on a dop stick with wax.
it didnt turn out good lol

[vitzitziltecpatl]

The speeds you'll use will vary according to what stage you're at. Just remember not to grind or polish on one spot for too long no matter what kind of opal it is. The heat differential between that one spot and the surrounding areas is often the real killer when opals crack during grinding and polishing. The ironstone boulder cuts more like any other rock, but the precious opal in the ironstone matrix can still crack because of this.

If your Andamooka has been cooked the dark color will go away if you take off more than a few millimeters. No worries, it can always be retreated. If you have matrix opal with enough natural color to be attractive without making the surface darker that's really good. Darker background color will always make for a flashier stone, of course, and if you want to do that you can treat it just before the final polish.

[hummingbirdstones]

Hi Aaron,

With the Andamooka matrix, it is made of either limestone or sandstone and is usually porous  -- the sandstone much more than the limestone.  A lot of people will stabilize the matrix before they cut it, some do it after if they want a high shine.  How porous your rough is will determine your ability to get a polish on it, hence the reason to use all diamond.  The powder oxides will get in the tiny pits and you'll never get them out.  Best advice is to use a lower speed to shape your stone as it will go quickly and you don't want to grind through the dark layer if it's been cooked.  If you have the limestone variety, it will take a better shine than the sandstone, but not a high polish.  Usually the best you can hope for with the sandstone is a matte finish without any treatment.

[lithicbeads]

 The rubber backing pad gives the lap some drape , the sanding surface bends around the contour of the stone to a degree. The unbacked diamond pad has no drape so is very good at getting out scratches that are of limited extent. All sanding methods have different drapes  and I have seen cutters who go from one type of sanding to another over sand on the second  because they are not sanding the stone carefully but rather they are sanding to the extent that they are relying on the drape curve to dictate the curvature of the stone. If you are going to cut the stone by relying on the drape to dictate the curve to a great extent it is much more efficient to stict with one sanding method.

[Bluetangclan]

You are SOL on the boulder, not familiar enough with Andamooka to comment on it. The whole treatment thing sends me the other way. Boulder will pit. Part of life. The better quality stuff will obviously not be as bad. Its not quite as bad as Honduran which I love working but far too time consuming in the summer for me to work it. I polish differently than Lithic. I do the pre-shaping on a 220 hard wheel. Final doming on a 400(I think) soft wheel. Do the 800. At 1400 I use a lot of pressure for about 20 seconds. Go to the 3k. On boulder I skip the moist 15k( I need to get hold of some slurry for regular opals). I use dry and very quick 50k and 100k buffer pads on a fast wheel, but dont let them get warm thus the quick part. Then a very light buff with not fresh zam on a slow wheel.

Boulder by its nature pits. It and Honduran are notorious for it.