How about a stone that "overlaps" with itself in a manner of speaking....
Someone showed me some beautiful crystals of jeremejevite from Namibia the other day - I hadn't seen any natural crystals before - these were maybe 2.5 cm long, a beautiful almost Ceylon blue with what looked like a beryl-like hexagonal prism habit. I read up a little on it and found some interesting detail:
Jeremejevite belongs to the orthorhomibic crystal system - though it shows a pseudo-hexagonal habit - which should make it biaxial. But faceted stones have shown a uniaxial negative optic character (texts variously list it as uniaxial negative and belonging to the hexagonal crystal system, with a note that it is anomalously biaxial with a very small 2v angle, also negative).
The differing optical properties are due to its growth zoning which is different for different localities: the optic character of the rim of the crystal is different than that of the core. From one locality the core is biaxial and the rim is uniaxial, but from another locality the core is uniaxial and the rim is biaxial.
So if you have the pleasure of studying a cut stone, one must keep in mind that the rim has been removed in the fashioning of it. It also goes to show that just one source book in your library is not enough.... and several sources might confuse one completely if you don't keep digging.
Joined: Tue Mar 25, 2008 8:37 am Posts: 15 Location: various
Hi all,
Casandra wrote:
Quote:
Jeremejevite belongs to the orthorhomibic crystal system - though it shows a pseudo-hexagonal habit - which should make it biaxial. But faceted stones have shown a uniaxial negative optic character (texts variously list it as uniaxial negative and belonging to the hexagonal crystal system, with a note that it is anomalously biaxial with a very small 2v angle, also negative).
The differing optical properties are due to its growth zoning which is different for different localities: the optic character of the rim of the crystal is different than that of the core. From one locality the core is biaxial and the rim is uniaxial, but from another locality the core is uniaxial and the rim is biaxial.
I took this picture of a jeremejevite interference figure.
What do you think of this sort of 'double center uniaxial'. Is it a' small 2V' figure?
Joined: Tue Mar 25, 2008 8:37 am Posts: 15 Location: various
Hi,
the conditions were 'normal' I guess:
crossed polars, strain-free glass conoscope
Casandra:
Quote:
the optic character of the rim of the crystal is different than that of the core
I remember now that in the Sinkankas 'Emerald and other beryls', the author mentions beryls (hexagonal hence uniaxial) with orthorhombic (hence biaxial) areas. I cannot give you more details as I don't have that book anymore. If anybody has that book, pages references would be welcome.
Now back to jeremejevite, here is another picture of an interference figure in the same stone as before (viewed from the opposite side as far as I remember).
Have you seen this type of figures before?
Don't you think it can be related to an 'anomalous optic character'?
Dear all,
About jeremejevite, I would like to inform you that and article and a ongoing study are available currently on GIA Laboratory Bangkok website in the Lab research part of the website:
The views expressed here are V. Pardieu’s opinions and do not necessarily reflect those of GIA Laboratory Bangkok (http://www.giathai.net)where he is an employee since Dec 2008.
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
A friend of mine, Elizabeth Price, brought over a ring she just had made with a 2.15 carat Portuguese-cut jeremejevite from Namibia. The stone is a very, very pale blue, very watery and has some hollow tube inclusions by one of the prongs,pictured below:
Some of the tubes seem to have a filler of some sort inside, and one tube (on the left) is bearded with euhedral crystals. Click the photo to enlarge.
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