How to tell if a sapphire table is perpendicular to the c-axis?

[LilAlex]

I feel like this should not be a hard question to answer but I can not find it. And I know that having the corundum crystal's c-axis perpendicular to the table is not the be-all and end-all for sapphire beauty. But is there a way to post-hoc determine the "orientation" of the native crystal material in the faceted stone? I don't know a dichroscope from a polariscope, etc.

 

[Bron357]

Hopefully I’ll make sense.
sapphires are dichroic so depending on the viewing angle, the colour seen is different. With blue sapphire it’s usually green (ish) or grey purple (ish) when viewing angle is 90 degrees to the c axis.This is easiest seen with a dichroscope. It can be suble or quite distinct depending on the gem crystal.
There are a number of factors when a gem cutter is deciding to facet a gem. Which orientation provides the best end size, best colour, disguises any colour zoning and excludes any major inclusions / crystal flaws.

 

[Rfisher]

I feel like this should not be a hard question to answer but I can not find it. And I know that having the corundum crystal's c-axis perpendicular to the table is not the be-all and end-all for sapphire beauty. But is there a way to post-hoc determine the "orientation" of the native crystal material in the faceted stone? I don't know a dichroscope from a polariscope, etc.

Interesting question!

assuming color zoning might get confusing (to uneducated as I) when looking via naked eye in some cases?
This is mine, just rotated ring 180deg. Same lighting/time of day.

 

[LilAlex]

^ Thank you -- these are good observations!

In my case, there is no visible dichroism at 90 degrees to the table (looking in "through the sides" of the stone). So I am still left with not knowing if there is only subtle dichroism or the orientation is suboptimal.

 

[VividRed]

^ Thank you -- these are good observations!

In my case, there is no visible dichroism at 90 degrees to the table (looking in "through the sides" of the stone). So I am still left with not knowing if there is only subtle dichroism or the orientation is suboptimal.

You need a dichroscope or enough experience with cutting gems.

I have neither of the two

 

[LilAlex]

OK, so it is not just the dichroscope but knowing how to use it. I see that they are all of $30 for a decent calcite one.

Richard Hughes notes (https://www.lotusgemology.com/index...96-pleochroism-in-faceted-gems-lotus-gemology):

"...When viewed in transmitted light with a dichroscope, pleochroism in uniaxial stones varies in the following ways...

• Parallel to the c-axis, only the ordinary ray is seen and thus there is no pleochroism..."

So if there is no dichroism when looking down through the table in transillumination, the c-axis is in the "correct" orientation.

 

[Starstruck8]

That's the theory...

I have a calcite dichroscope. I tried this on my sapphires, looking down through the table. I couldn't see an effect like those in Hughes' figures 5 or 6 in any orientation. Maybe my sapphires are too small. Or maybe I need to tune my eyes.

By contrast, it was easy to spot dichroism when looking sideways through the dome of a star sapphire.