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Chalcedony or hyalite opal?

Posted by Duncan Miller  
Duncan Miller March 20, 2017 11:55AM
A friend of mine sent me these photographs of a specimen from the 'blue lace agate' mine at Ysterputs in southern Namibia. It consists partly of what are thought to be blue chalcedony pseudomorphs after fluorite, with a surface coating of semi-botryoidal clear material that fluoresces yellow-green in short-wave ultraviolet light. Her question is how to distinguish between fluorescent clear chalcedony and hyalite opal.

Reiner Mielke March 20, 2017 01:21PM
You can scratch opal with a good steel file but not chalcedony. Test your file on a piece of feldspar first. It should be able to scratch the feldspar, some cheap files will not.
Duncan Miller March 20, 2017 06:20PM
Thank you Reiner. Your suggestion cleared this up quickly. "With a bit of a sawing action with a fine steel file, I could make a groove in the feldspar, but couldn’t in one edge of the chalcedony, and am reluctant to try somewhere else on the specimen." So it appears to be clear chalcedony, not hyalite opal. Duncan
Alfredo Petrov March 20, 2017 07:28PM
I agree with this being chalcedony. But I hesitate to use hardness as the proof for distinguishing hyalite from chalcedony - Both of them show a range of hardnesses and both can be around 6.5.
Better proof would be to heat a fragment in a pyrex test tube and see how much water comes off. Or check the refractive index. If you take an optically dense liquid (even sugar syrup will do), and drop in a sand-size fragment of colorless chalcedony and a similar size piece of hyalite opal, the difference in refractive index is obvious even to the naked eye; no refractometer needed.
Reiner Mielke March 20, 2017 08:43PM
Hello Alfredo,

What you say may be true, however if you cannot scratch it, then clearly it cannot be opal. If he could have scratched it then it could be either a soft chalcedony or opal.
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