Aussie Lapidary Forum

LAPIDARY => Ask a "Silly" Question => Topic started by: JLB on December 20, 2008, 09:07:26 PM

Title: Terminology.
Post by: JLB on December 20, 2008, 09:07:26 PM
Don't all rush me now   ;D    A kind authority gave me some definitions of what a window in a sapphire was.  Today on rereading the e-mail, it eventually sank in. (I think).

IE. A bad cutting on the pavilion allowing vision straight thru the crown or table, with out the usual reflections back thru the gem.     And also used where the colour runs out a bit.

Any more examples ??   

I also wasn't inundated by corrections to my guess at what "silk" would look like.  I suggested
a foggy area, but now I'm wondering if it may not be like "threads".    Comeon now, I know you all take your laptop on holidays with you.

When I get my silk sapphire, I want to know what to look for, and not panic like when I couldn't find my promised window.   ::)

jlb.
Title: Re: Terminology.
Post by: Aussie Sapphire on December 21, 2008, 07:32:48 AM
Gday John,

For the full story on windows in gems, see this article (link below) by Richard Hughes - he is a world authority on sapphire/ruby and has some great articles on his website.

"Brilliance, Windows and Extinction in Gemstones" (http://www.ruby-sapphire.com/brilliance_windows_extinction.htm)

(http://www.ruby-sapphire.com/images/brilliance_windows_extinction/window-2-gem-blue.jpg)

You can see in the photo (sourced from the article) how this works in practice.  The windowed gem on the right is just not as attractive due to poorer light performance.

In your case, the term was used in a different sense - more on this in a moment.

cheers
Leah

Title: Re: Terminology.
Post by: Andrew on December 21, 2008, 08:41:11 AM
Hi all,

You are on the right track for sure - a simple idea to think when working out what a window is in a Gem is to simply go back to the Glass in your nearest Window in your house. In a Blue Sapphire the term "window" is often used for a Clearer area in the Stone. This can be caused by a poor shallow cut, incorrect orientation of the stone for cutting but very often just a natural feature of the Gem.

Rough Sapphire with this Clear area can sometimes be tilted when preforming to disguise the lighter area - the window will only then be possible to see as the cut Gem is tilted in exactly the right angle - face up it looks fine.

Badly windowed Gems are common in some cheaper online shops and on Ebay for example.  They receive huge discounts in the trade and are therefore a common stock item for resellers without the funds for better quality stock or knowledge about Gems.

Silk is a common feature in Sapphire and Ruby and will mostly be tiny needles of rutile.  In some cases, it is actually desirable as in true Kashmir Sapphire where it can produce a soft "Sleepy" appearance that makes you almost want to jump in with the stone. 

Heavy presence of silk is also necessary for star sapphire where the needle inclusions are aligned in just the right way to split the incoming light to produce a star pattern of light rays. Some of these can be beautiful indeed - in fact I have just added a very nice new star to our collection and will try to post a photo soon.

Some Silk has also gained acceptance from consumers as it is a good sign that the Gem is Natural and not man made, heavily "burnt" or altered.  JLB - your description of Silk as "a foggy area" gives a easy to visualise description - Cloudy is also used.

Many of the old basic heat treatments that have been well accepted and routinely done in the industry for over 40 years are aimed at dissolving silk in order to leave a clearer, brighter gemstone.  Some people think of it as finishing the job started in the volcano when the corundum crystal started to form - some natural crystals have had the right conditions to be perfectly flawless inside while others have had slightly cooler growth conditions allowing these rutile (and sometimes other minerals) needles to form.  Heat treatment (sometimes referred to as burning) will usually dissolve the rutile needles and make the gem brighter.

 Hope this helps,

cheers Andrew
Title: Re: Terminology.
Post by: JLB on December 21, 2008, 02:43:55 PM
Thanks a heap, Leah and Andrew, I know you must be busy.
JLB  (John.)
Title: Re: Terminology.
Post by: JLB on December 22, 2008, 12:48:22 PM
A bushie type just told me another definition of a window, the only one he uses, is where he cuts a spot on a piece of rough, to look inside for flaws etc.

Andrew's link was terrific, even learning me about my next silly question.   A certain sapphire I have looked a bit funny to me only having seen immages and gems with the usual cut pavilion.   I went to the trouble of sitting down and drawing this strange shape working from girdle to culet to try to make sense of it.     

The said link just happened to have a similar drawing.  Just a step cut apparently.    Only mine has four steps.  His drawing only had three.  His bad luck.    ;D