is this an Opal?some idea on what is it?

[greengems]

I get this stone one year ago;I saw all the process
from the finding of the rough in rainforest river in Sumatra
til the cut,grinding and end polish with diamond powder
We thought could be a Milky Opal but without a proper
test could not tell 100%
Someone has seen some similar stone?some ideas
on what is it?

Best regards

 

[Richard M.]

We thought could be a Milky Opal but without a proper
test could not tell 100%
Someone has seen some similar stone?some ideas
on what is it?

greengem, as a field collector and cutter with many years' experience, I'd say it looks very much like an agate nodule with fortification banding. Some nodules of that type also contain common opal but without having the stone in hand it's impossible to know whether this one does.

There is much more "common" opal (opal without play of color) than the very desirable precious opal (with POC). Common opal can occur in many colors, especially white. Currently pink and blue opal from Peru are quite popular, as is the fine green opal from Africa. I have lovely green common opal from Macedonia. It also occurs in black, gray, brown, yellow, orange and red (Mexican Fire Opal) and other colors.

This is an image of a faceted yellow "common" opal from Idaho, USA, that I sold last year:

 

[LD]

I agree, this looks very much like an Agate. I make my own jewellery and this type of agate which is brown, black, cream, grey and sometimes orange etc., is usually called Botswana Agate. It's very pretty.

 

[greengems]

Thanks for the replies
I will try to make a photography of some rough from where it was cut this cabochon that i still have and post it.
I say milky opal the material white in the center of the cabochon,is like a blurred white;the rough is like keramic,different from agates I have seen;I will try to upload some photos of rough
Wow that "common opal" faceted id gorgeous!!!
I have seen opals in Banten,in Java that they call Teh Kalimaya and toher type is black colour with all the characteristic rainbow colours display,the black one is really something unique,I think can compete with best australian opals;just make a google search by name of Kalimaya Banten and Kalimaya Teh and Kalimaya banten hitam
They call it Kalimaya to Opals in Indonesia,Kali menaing river and maya is an area of banten as the roughs are found on what used to be a river called Maya River(nowadays dried)
Thanks very much to all

 

[Richard M.]

greengem,

Yes, Indonesian black precious opals (with POC) are indeed beautiful. But unlike the famous sedimentary black opals from Lightning Ridge, Australia, they derive from volcanic processes and tend to be much more prone to crazing for some reason. That's the pattern of volcanic opals around the world for reasons that aren't yet clearly understood. LR blacks are well known for being stable and very unlikely to craze -- and that, along with their incredible beauty -- is among their strongest selling points.

That's not to say all Indonesian black opals will craze. But as a potential buyer you should be aware of that possible downside.

Richard M. (Rick Martin)

 

[Richard M.]

greengems,

I forgot to thank you for your comment on the yellow "common" opal. Actually, common opals are not all equally "common." It's just that gemologists arbitrarily separate precious opals (those with play of color) from types that do not, which fall into the"common" category. When non-POC opals possess fine color and good clarity they are highly valued for those qualities.

Red fire opal, principally from Mexico but also from several other locations, is an example. The yellow opal I posted also falls into the "fire opal" category, as do quite a range of colors: amber, orange and orange-red.

Richard M. (Rick Martin)

 

[greengems]

Thanks everybody for replies
This is some of the same rough from where that cabochon was carved from:

Best regards