Why does this blue sapphie have such a zoning?

[haagen_dazs]

0.85 ct Sapphire

Hi all
I was just wondering if its normal for sapphires to have such a zoning? Is this even called zoning? The whole pavilion is blue and the crown region is colourless.
However the description says that the face up is all blue ("colouration complete") and actually looks pretty nice.

I normally do not use corundum cut in emerald shapes so this was interesting.

 

[morecarats]

Natural corundum is colorless. It is colored by impurities, such as titanium and iron. If the impurities are not evenly distributed through the material, you can get extreme color zoning. Usually the badly zoned specimens are not displayed in polite company. But they can be shown to students for educational purposes.

(As my Thai grandmother has been heard to remark, "Oy, such a zoning!")

 

[haagen_dazs]

Date: 12/22/2009 11:38:44 PM
Author: morecarats
Natural corundum is colorless. It is colored by impurities, such as titanium and iron. If the impurities are not evenly distributed through the material, you can get extreme color zoning. Usually the badly zoned specimens are not displayed in polite company. But they can be shown to students for educational purposes.


(As my Thai grandmother has been heard to remark, ''Oy, such a zoning!'')

is this sapphire considered a badly zoned specimen?
what do you think of it?

 

[T L]

It faces up okay, but the zoning is pretty apparent through the pavillion. They cut it to minimize the extreme zoning so it would have the least affect into the table. It''s really too zoned for me, I prefer much more even color. However, the price reflects this. It''s inexpensive, but I even think it''s a bit too high in price for that size and quality.

 

[Largosmom]

Even in the expensive stones I was offered at my local jewelry store for my ering the stones had a lot of zoning, but not this extreme. I didn''t like it much as it made the stones look uneven to me even if looking directly down, they were relatively even, if you looked at other angles, portions were quite colorless. I *think* they were overpriced from what I''ve seen here, but I still haven''t louped enough to say that with any assurance. The best of the bunch, which was over $3000 for a 7 and a bit mm stone was the least zoned.

Laura

 

[chrono]

Zoning is a common thing in many rough material but many are cut to mimimize these effect. The worst of the lot are never sold, or if they are, they might be in a lot or parcel. This particular sapphire is quite well done to still show a good blue face up. It might be a fun stone to keep but not something I would get as a special piece.

 

[Michael_E]

Date: 12/22/2009 11:17:17 PM
Author:haagen_dazs

I was just wondering if its normal for sapphires to have such a zoning? Is this even called zoning? The whole pavilion is blue and the crown region is colourless.

As sapphires are forming they are surrounded by hot, high pressure fluids from which they crystalize one layer at a time. The coloration or silk inclusions in different areas of the stone are due to those areas having more or less of the chemicals that make up the color or silk inclusions. Because of this a sapphire, (or any crystalline gem), can have color or silk inclusions in clouds and stripes or other shapes. When a stone is cut which has color in only one place, that color is usually placed so that it''s in the pavilion of the stone so as to optically distribute that color throughout the stone. Color zoning and or silk are extremely common in sapphires, even if they are hard to see without magnification.

One thing to keep in mind with blue sapphires having a mostly clear area in either the crown or pavilion is that this is the most common trait of flame fusion synthetic sapphires. These synthetics have great color and are commonly sold via Ebay as natural stones, so you''ve got to watch out. These synthetics come in rough form as a split boule which looks like a carrot which has been split lengthwise. The color is confined to the area around the perimeter with the remainder of the stone being nearly colorless. These stones, when cut, do not have color zoning, but do have growth lines which are different from natural stones in that the growth lines are curved and only noticeable under magnification.

The stone that you''ve shown has growth lines which are straight and color zoning which indicates that it is natural. Those lines may or may not be noticeable when the stone is in hand. This is because you have two eyes and your brain blends the two views that each eye gets of the stone, causing many inclusions to be less noticeable than they are to the camera. As an example of natural color zoning and silk I''ve attached a picture of an Autralian sapphire which is slightly silky and distinctly zoned. This effect is totally invisible face up, though the stone does look a little "foggy" when viewed in strong lighting.

 

[MakingTheGrade]

Thanks for the great info as always.

 

[haagen_dazs]

thanks michael!
that was really educational!
its neat that people with experience can tell if the sapphire i linked was natural
cool

 

[deorwine]

Michael E actually cut me, several years ago, a beautiful (round brilliant) natural sapphire with severe zoning such that something like a quarter of the stone is colorless (the rest is blue), and if you look from the pavilion side the demarcation is extremely obvious (like, really, it looks like a colorless chunk attached to a larger blue chunk). However, because he''s an experienced cutter he was able to orient/cut it so that face up the whole stone looks blue, and you would never know that it has such severe zoning unless I told you or unless you tipped it at the right angle.

(And the best part was, he let me have it relatively cheaply, as his rough cost was cheap because of the zoning -- which was how I ended up with that particular stone; he was working with my being a student at the time and having a quite restricted budget. A super guy!)

Maybe after Christmas I''ll see if I can get shots face up and pavilion up, because I actually think it''s pretty cool.